Float Line

I'd say 50 feet should be plenty unless you want to plan ahead and think you will be greatly increasing your depth. But float lines are a general pain in the ass to deploy out of the boat and then bring back in or take over the rocks, so more length than you need is just more hastle.

BTW, the reason for my wise-ass reply last night was that if you happened to be diving in California kelp with no float on the line, then my answer would have been entirely different. It may seen counter-intuitive, but you need a line much deeper than the depth of the water because you don't have a float to fight the fish and keep the line from being taken away from you, and the fish can go under kelp and then run horizontally while you are being pulled vertically.

For instance, a friend of mine shot a 60 pound white sea bass at Santa Cruz island in just 25 feet of water using a 100 foot float line. Even though the line was so much longer than the depth of the water, he had the end in his hand and was being pulled straight down, struggling back up to get a breath now and then. He was on the verge of turning loose when the boat arrived and he was able to grab the swimstep.

I suppose that in Hawaii a fish could take your line under a ledge and run horizontally with you being pulled down vertically, but you would be using a float big enough to fight the fish. And I'm told that state law in Hawaii requires a float anyway.