Re: 1981 Aquasport 22 FF Rebuild and Convert from I/o to Outboard
US Comp polyester resin, CSM & 1708 fiberglass.
Marine grade is a specialty plywood $95+/- PER sheet of 3/4". It's factory has been or the sheets themselves have been certified by a quality control inspector to be made w/ minimal (or no) defects (if present all repaired & small) in the face laminates (the outer layer of wood veneer on either side of a sheet), no voids (or minimal/small) in the core laminates & no voids along the outer edges of a sheet.
Go into a lumber yard & look at a stack of plywood. On either or both sides you'll see knot holes (w/ no wood), football shaped repairs (often multiple/large repairs), and lots of areas along the edge of the sheet where there is no wood (voids). Grab a random sheet & cut it into 6, 8, or 12" squares & the percentage of freshly cut edges that also have a void will likely be high. No way for you to determine if there are voids present out in the center of a 4X8 sheet of plywood, until you drill or cut into 1.
Generally, ACX exterior (that's the X in ACX) plywood will have few knot holes or repairs on 1 face (side) of the sheet (that's the A in ACX) and is an acceptable alternative, IMHO, that will cost @ $35/sheet of 3/4". It is made w/ waterproof glue, and is generally a better grade of lumber then a run of the mill sheet of exterior plywood, which is often a BCX grade....
MDO & Auraco are often used as well. You might be able to price them at your local lumber place, the price depends on where you are.
I can get or order ACX all day at lots of places, in any thickness I want, at no extra cost beyond what a sheet costs
MDO I can order in at a few places in any thickness I need: 1/2, 5/8 or 3/4", no extra costs but I have to wait 5-10 business days, most places that can order it, only have 1/2 or 3/4"
Auraco was only available in 1 location, 3.5hrs from me, and is no longer available, not even as a special order
Marine is available in 3/4" at 1 store, 90min from me, at $95/sheet, or about the same cost, but via special order from a couple places closer, but w/ a 5-10 business day delay.
Is marine grade better then ACX, sure it is, but is ACX insufficient? Nope, both have to be encapsulated in resin & glass. If a money stream was unlimited, I guess most would use the marine grade, but it's not readily available when you need another sheet. Many of us don't rehab old boats because we really want to, we do it because to have a nice boat, we need to rehab an older 1 for much less then the cost of a new boat. Would $95 a sheet for marine break the rehab? Nope, but if you can buy $35/sheet and accomplish the goal equally as well, why not. Easy of access is my primary motivation for recommending the ACX.
Check for MDO &/or Auraco too though, both are less then marine & really great to work w/
Good luck.... ^^^^sorry about the long winded reply...