Fishing is a controversial skill: Some like it, but most try their best to avoid it. So I guess I should tell you why fishing is worth it first:

There are some decent money making methods that are locked behind a fishing skill requirement. That includes Grim Reapers during Spooky Event (800k - 5m per), Great White Sharks during the Fishing Festival (0 - 3.2m per), Yetis (0-17m per) and also the Soul Whip, which is not exactly required to fight Ghosts - but makes it a lot easier. To be able to do all of these you need at least fishing 26.

And then there also is Lava Fishing (on the Crimson Isles), where you can get Magma Lord Fragments or even Radioactive Vials, plus a bunch of salvagable Fishing Gear.

Picking the right Gear is quite easy: Always get as many pieces of the highest gear you have unlocked. There are no choices to be made here, except maybe for pets.

Fishing 0 Angler Armor

Fishing 13 Salmon Armor + Water Hydra Head

Fishing 20 Diver Armor (replace Salmon boots first, helmet last)

Fishing 24 Shark Scale Armor

Fishing 27-35 Lava Sea Creature Armor (Slug Boots, Moogma Leggings, Flaming Chestplate, Taurus Helmet, pieces have different fishing requirements)

Fishing 36 Thunder Armor (you need to fish the 24 Thunder Shards yourself)

Fishing 45 Magma Lord Armor

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Fishing 0 Fishing Rod

Fishing 6 Speedster Rod (alternatively get Ice, Sponge or Prismarine Rod before)

Fishing 10 Challenging Rod

Fishing 15 Rod of Champions

Fishing 20 Rod of Legends

Fishing 24 Rod of the Sea

(Fishing 25, Yeti Fishing only: Yeti Rod or Auger Rod)

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LAVA FISHING (other rods can't be used for lava fishing and vice versa):

Fishing 10 Starter Lava Rod (don't start lava fishing with fishing 10)

Fishing 27 Magma Rod

Fishing 30 Inferno Rod

Fishing 35 Hellfire Rod

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Now the only pet you need at first is gonna be a dolphin pet of any rarity you can afford. Later you can get a squid pet as well, to get more fishing xp. The preferred way to use it is by using auto-pet rules:

Rule 1 On throw fishing hook: Equip Dolphin

Rule 2 On enter combat: Equip Squid

That way you get the bonuses of both pets. Please note, that for this to work you have to kill the Sea Creatures with your sword. Otherwise it will just swap the pet once per throw, alternating between them.

If your mining level and hotm level is high, you should use the Ammonite Pet instead of the Dolphin pet. It has only 4 less Fishing Speed than Flying Fish but comes with almost as high of a Sea Creature Chance as Dolphin (Mining LX (60) and Hotm 7).

The mythic Flying Fish not only gives a lot less scc than the ammonite pet, but is worse than it atm, since the ammonite pet is bugged to give more fishing speed than it's supposed to. Insted of giving 0.6 per mining level (max of 36) it gives 0.6 per mining and fishing level (max of 66), while the flying fish only gives 50.

DETAILED GUIDE

FISHING STATS

First lets talk about the generic stats that have an effect on fishing.

Combat stats like weapon damage, Crit Chance, Strength, ... help you kill Sea Creatures faster - obviously. Later on you should be able to consistently kill Sea Creatures with your fishing hook, so you don't have to switch to a weapon.

Magic find

When fishing up legendary Sea Creatures you will want to get the best chance at getting whatever rare drop it has. So you want your magic find to be as high as possible. A common strategy used for that is to switch to Sorrow Armor and a pet that gives Magic find before killing the sea creature. Theres also some good drops on weak mobs, like Carrot Kings (should you use Carrot Bait), Werewolves or Deep Sea Protectors. Since you will instakill them at one point, consider using the lucky reforge (Lucky Dice) on your fishing rod, which gives extra magic find.

Main fishing stats:

Fishing Speed

Fishing Speed does the obvious thing: It increases your Fishing Speed. 50% more Fishing Speed means the catch time is halfed. You get Fishing Speed mostly from your rod, but the Fish, Minnow and Whale Bait give a Fishing Speed bonus. The Flying Fish pet, Dolphin pet and the Ammonite pet all increase the Fishing Speed, with the bonus of Flying Fish being the highest. Armor doesn't directly boost the fishing speed, but if you want to boost it put Legion on all Armor pieces. It might be worth putting it on shark scale and thunder armor, however you may want to upgrade eventually and in case of thunder armor the legion books would be completely wasted. If you have Magma Lord Armor don't waste any time in putting it on.

Sea Creature Chance (SCC)

This one is probably even more important than Fishing Speed, because as the name suggests this is the chance that you catch a Sea Creature. More Sea Creatures means more money and more fishing xp.

You get SCC from your fishing armor, your rod, enchants (Expertise and Angler).

Other ways to get SCC include the submerged reforge (Deep Sea Orb), any fishing rod reforge, Legion, Dolphin or Ammonite pet with a Washed Up Souvenir, a T5 Beacon, SCC enrichment on all of your legendary talismans and the Sea Emperor Cake buff active (blue century cake).

Treasure Chance (Treasure Hunter)

This is a minor fishing stat. You can only level it up by increasing your fishing level. It increases the chance to get a good or great catch.

FISHING STRATEGIES

EVENT FISHING

One of the most profitable way to fish is during events, however to really make money from these you already have to have fishing level 24-26. But the Season of Jerry, Spooky Event and Fishing Festival at the very least add some variety and in case of the Fishing Festival also help you get xp faster. Quick guide:

Season of Jerry:

Fish in the Jerry Pond using your best rod, later on use yeti rod/auger rod. This is available during all of Late Winter, so 10h per skyblock year. Main money source: Baby Yeti pet from Yeti sea creature, requires fishing 25

Spooky Event:

Fish anywhere starting 3 skyblock days before up until 3 skyblock days after the Spooky Festival. Main money source: Soul Fragments and Vampire Fang from Grim Reaper (fishing 26) and Deep Sea Orb from Werewolves, Phantom Fishers and Grim Reapers (at least fishing 17)

Fishing Festival:
Only when Marina is mayor on the first 3 days of every month, Main money source: Great White Shark Teeth and legendary Megalodon pets from Great White Sharks (fishing 24), also epic Megalodon pets from Tiger Sharks (fishing 18)

HYPE FISHING
This is the dominant strategy for high level fishers and as the name suggests you need a Hyperion or another Necron's Blade for this. Select mage reforges (that will probably be Sighted (Ender Monocle)), put necrotic and loving on your armor and then start to fish. Whenever you get a sea creature simply hype spam until it's dead.
If you have enough sea creature chance you should think about switching your helmet for Wither Goggles to kill the mobs faster.

ENCHANTS AND REFORGES

For your Armor there's not much you can do at first, eventually you will end up with recombed legion 5 Magma Lord Armor.

The reforge on it is a matter of preference. You could use submerged (Deep Sea Orb) to get closer to 100 scc without the need of scc enrichments and legion. If you're hype fishing you could also choose necrotic/loving to do more damage and to not run out of mana (most popular option). Renowned (Dragon Horn) is also an option if you want to have a bit of everything (magic find, health, defense, intelligence and sea creature chance can all be useful when fishing for jawbuses).

You can get almost all of the t6 enchants for your fishing rod immediately, since they are very cheap. It's only really worth it once you got a Challenging Rod/Magma Rod though, and only get Caster VI (6) if you use expensive bait like Whale Bait. The best reforge at first is Salty, which can be applied at the fisher hut near the Jerry Pond (only during Season of Jerry) for 64 Hunks of Ice. If you want extra magic find on your rod you can get the Lucky reforge (Lucky Dice), if you want a cheap alternative to put on your rod when there is no Season of Jerry get the Stiff reforge (hardened wood). The only thing missing now is Expertise and your rod is pretty much maxed.

For Magma Rods also pay attention to your attributes. Before upgrading the rod to an inferno rod you should make sure to decently upgrade the rods attributes by combining them with other magma rods with the same tier of attribute. Once you have an Inferno Rod you're gonna have to use the more expensive attribute shards to upgrade attributes.

The best attribute combinations are

Double Hook/Fishing Speed for normal fishing

Trophy Hunter/Fishing Speed for Trophy fishing

BAITS

When Fishing you'll always wanna be using some kind of bait. A bait thats good in every situation is Fish Bait, it makes fishing faster and thats it. For Fishing Festival and Spooky use the corresponding bait (Shark and Spooky Bait) - unless you have some coins to spare, in which case you should pretty much always use Whale Bait. For Trophy Fishing use Hot Bait, it will help you a lot.