If you can find it locally, and it fits the budget, yes marine ply is best. I can't even special order it from any of the local lumber yard here in WV. I've seen it once at an HD 3/4" 4X8 sheets $95/ea, 1.5hrs from the house, but the next time I was there, none left and no plan to order/get any more.
Arauco (if you can find it) is a great choice, but hard to find in some places
MDO (Medium Density Overlay (
not MDF, the carp wood chip ply used in cheap put it together yourself furniture))
Many use ACX (good grade of exterior ply) because they can get it off the shelf at most big box hardware/lumber places )HD or Lowes, etc)
last resort, BCX
ACX ply has a 'good' face on 1 side (that's the A in the ACX) and a lesser grade face (the C side) and is exterior rated (the X in the name).
No matter which you choose, they all have to be sealed w/ polyester resin and fiberglass mat/cloth and if neglected they will all eventually suffer similar problems as your current deck. Which is why many use ACX. $30-35/per 3/4" 4X8 sheet.
You can build the seat bases, but how much of the original seat will be reuseable?
Might consider new fold out lounge seats, which come w/ bases. You can find them online (iboats does carry them) and they often have plastic or composite bases for $140+/- for each foldout/lounger upto $4-500 per seat.
If the seats need to be rebuilt, but the vinyl can at least be used as a pattern, you could build the bases and sew the vinyl yourself to fit the existing. Lots of people
iboats have w/ great results.
Here's WoodOnGlass's graphic for rebuilding the boat:
Fabricating Decks, Stringers, and Transoms