Regular dragons are therefore Heavensward. The new heat? Sea Dragons. Sea Fishing in Final Fantasy XIV gives you the opportunity to exchange your spear for a fishing rod and earn the title "Sea Dragoon"; but only if your party can catch 100 sea dragons in one fishing trip.
This guide explains the optimal strategy for earning the "Maritime Dragonslayers" fishing bonus, which allows you to achieve the "What Did Sea Dragons To You?" Achievement.
Before fishing
First things first: Unlock Sea Fishing. Equip your level 80 fisherman with GP-fused Facet gear and grab a stack of Hi-Cordials. You'll want to have at least 800 GP before trying this achievement so that you can use Identical Cast and Double Hook to catch multiple dragons at once. Bring a stack of Ragworms for the Shaggy Seadragons you are going to fish.
These fishing achievements require a group of like-minded fishermen. Spectacular weather while fishing for dragons will likely sink your run. If you need help finding a Static Peach, check out Party Finder before the next fishing windows. There are also a few FFXIV Fishing Discords and Cross-world Linkshells full of avid anglers; Limsa's Shout Chat is a great way to connect.
Fishing routes
Check out the sea fishing sailing schedule by speaking with Dryskthota at the docks of Limsa Lominsa. For seadragon achievement, the best route ends in the North Merlthor Strait at sunset, but you can catch dragons along any fishing route that stops in the South Merlthor Strait. .
How to catch sea dragons
When in the Southern Strait of Merlthor, use ragworms as bait. Shaggy Seadragons are a small tug (indicated by an exclamation mark) and tend to bite between 12 and 15 seconds after throwing. The blind double crochet here is risky. Sea Bombs share the same time frame as Sea Dragons and also have a small tugboat. Once you've caught a Shaggy Seadragon, use Identical Cast and you're sure to double the hook. Avoid reeling fish with medium (!!) or large (!!!) tugs; you can trigger a spectral window.
If your party ends up in just under 100 after your stopover in Merlthor and you're on the Northern Strait route at sunset, you can try to catch a few Coral Seadragons under a spectral window. Use Ragworms to grab a Spectral Sea Bo and cross your fingers for the Spectral Weather to appear. When spectral currents are on the rise, keep using ragworms to catch coral seadragons. Watch out for small tugs (!) That bite right away. There are a lot of layers of luck needed here, so don't rely on this method to get over 100.
Special thanks to success hunter S'yahn Tia from Cactuar for compiling tons of fishing data into one spreadsheet.