well I have a friend who made one and he is unpierced so far...
From my viewpoint, for a long lasting, low maintenance tub, I would only consider two options, fiberglass or pond liner. For quick and easy, a fish safe, type pond liner would be the best option.
thank you all for your replies. I have decided to go with marine grade ply (at $140 a sheet) and probably won't paint the insides at all now.
You will need medical tweezers to pull the slivers out of your backside.
Marine ply is 7 to 9 cross sheets as opposed to regular ply of 5 for the same thickness. That makes marine ply stronger but each ply sheet is much thinner and no better at holding lamination then regular plywood.
That thinner ply material will pierce your hide far more exquisitely.
I cringe for you Now !!
$140 a sheet? Wow, A-B Marine grade plywood sells for about $80 for a
3/4" x 4' x 8' sheet at my local big box retailer, (Home Depot.)
Truck bed liner paint perhaps? Although, as noted, plywood is an likely an unsuitable material, since it tends to delaminate in the constant presence of water (marine plywood maybe?). In any case, since your quesiton was about paint, I can't think of anything more robust than truck bed liner paint.
Redwood, Cedar or Oak for water held in the wood.
Plywood may de-laminate even with a liner material.
Why would you paint the inside. You'll need some sort of plastic, resin or fiberglass liner inside if you want it to hold water.