I put a new floor down in Mad Mikey's drift boat back in June, used something like 15/32 or some crazy thickness was little over 1/4 inch his Stringers that it sat on was only about 12 inches apart. I walked around on it and was ok at the time was about 270 LB talked to him couple days ago and he said it was holding up real good he has been useing it almost every week end.
I just replaced the floor in my driftboat too. I upgraded from 3/8" to 15/32" (which some lumber yards call 1/2") because I wanted a sturdier floor. My stringers are about 12" apart and it's really solid now. Make sure you use exterior grade plywood (ACX).
i used pressure treated 1/2" plywood in my driftboat and covered it with marine vinyl. i got the plywood at home depot. its about $25 for a 4x8 sheet. i have it supported about every 12 inches and its very strong.
since its my first post, here's my story. i moved to oregon about 4 years ago and got the steelhead/salmon bug. i've been fishing ever since. i started building a 17.5' aluminum drift boat about a year and a half ago and i'm just about finished. all thats left is painting the exterior.