Build a raft to hunt these things, by placing pillars towards the front about 2 or 3 high, and a ballista on some ceilings. You should be high enough that you can shoot over the raft driver's head and hit the pursuing fish. The raft driver should zig zag to avoid the fish. You can kill it with arrows or other means as well, but a ballista is much safer.
It will attacks rafts so be carful coming across one on a boat.
Also the Alpha Albino Leedsichthys Will give you raw prime fish me when you scrape meat off of it, and killing it will give you The Captains Hat Skin