depends where the timber is...and usually cd ply will do the trick just as well as marine coz the glues are similar...either way u need to seal it whatever u use. Freshwater will kill ply faster than saltwater due to the rot so u are just as much protecting the ply from yourself and the hose. No idea what a tabs territory looks like or where the wood is but in general the best waterproofing u will get is epoxy...if its an important piece in a wettish area its best to actually fibreglass it to last.
What u generally want is for the first coat of whatever u use to soak right into the ply.
One of the best ways is to get a tin of good old polyester resin and a tin of what is called styrene monomer from the same supplier. Mix them about 50/50 and be heavy handed with the hardener..say 3-4%MEKP . It will be a very thin resin mix for the first coat and u will find it soaks right in. Then another coat with only about 20% styrene then lastly none at all.
Epoxy is harder and more waterproof and u can get already thinned down stuff like "everdure" from the average chandlery (not cheap tho)...does a decent job but u still need multiple coats and follow the directions re application. In a wet area i'd go the epoxy especially for decking and it will last for eons. Beware tho u might need a specialist flowcoat over epoxy (if u want this as your final finish) as the regular polyester flowcoat doesnt stick to it so well unless u can get a good mechanical bond with heavy sanding. Its one reason i tend to stick to polyester even tho its not as good at waterproofing...coz i can just flowcoat straight over it.
ANY type of ply can be used in reality as long u seal it very well....i;m even using 6mm bendy ply which doesnt even have a waterproof glue and is soft as hell...after shaping and securing i just hit one side with multiple coats of thinned resin then let it harden into shape...then take it off and do the other side...then flowcoat or upholstery for the colour.
There are easier ways of course...but everything else is a short cut and it just depends how long u want it to last and how much water it cops.