Building Oyster barge (work barge)

2 1/2' is a wasteful size for plywood. 2' works well and you could put a removable board around the edge of the float for waves etc.
Sunk half way, 1 foot, it would be displacing 7,700 lbs.

$8,000 sounds like an awful lot of money for something like that.

 

I'm pretty sure one can get 4x10', 5'x8' or 5'x10' ply in exterior grade. 5'x5' (actual) Baltic Birch might be lots better material than regular exterior but it would represent more seams to scarf. Both are more expensive than 4'x. Is that why cost is high (I'm going with Sam's assessment of the cost)?

On an added note: do you know of a source making 5'x marine ply? That could be potentially cool to know about.

 

He's in the People's Republic of New England. Everything is more expensive up there.

I still don't understand the need/want for the high deck. Even with only 2 1/2' , that is a lot of volume/displacement. Are you planning on using it as holding tanks/bins for processed critters?

 

This sounds like a project for those plastic drums, that many seem to want to use to make houseboats etc. Or maybe you just want a platform supported by stilts on the bottom, depending of course on tides range and water depth.