The Legendary Fisherman has never been considered competitive, but because I like the monster so much I've been trying to come up with the most competitive version of the deck possible without relying on other archetypes.
The main strategy of this deck is field control. Use your resources to control the field until you can bring out your more powerful monsters the level 7's. Combined with Sea Stealth Attack, the level 5 & 7 monsters are a nuisance. An ideal field is to get A Legendary Ocean, Sea Stealth Attack, and 2 water monsters on the field doesn't really matter which.
I would recommend at least 1 or more Gameciel in the main deck if you can figure out what to take out for it.