Waterproof 18mm panels

There's Stokboard, I've used it for trailer floors and it's OK. Probably not quite as stiff as ply so may need a bit more support.

However, what sort of boat is it? we have a canal boat in France which has plywood flooring which is well beyond ten years old and still quite sound. Why are your floors so damp?

Phenolic resin plywood? I have an offcut which has survived outside for years.

Buffalo board?

This question interested me so I spent some time looking Yes buffalo (phenolic resin coated plywood) board is probably pretty good. And comes with a non slip surface.

You would probably want to coat the non coated surfaces in something like epoxy resin as well, and maybe add glass cloth as well

Have you considered a recycled plastic board, such as

I've got a trailer that could use a new bed, might be suitable ...

Bugger thats 255 quid a sheet!..

'Boats never were cheap...

Generic (rather than buffalo) versions are available from other suppliers.

That's a B&Q 'marketplace' listing which includes national delivery. Those are a waste of time - if you want Amazon/ebay style listings then use those sites instead. Better to go direct to a supplier who will deliver locally.

eg this one:

Theo

Many thanks for the replies and thoughts, greatly appreciated.

After talking to a supplier of panels my understanding is that all plywood is liable to ingress of moisture and I leaned that WBP is an old term and no longer used.

Even marine ply faces must be coated. The best bet is a phenolic resin faced panel, aka trailer or buffalo board, where the cuts must still be painted with a rubber or other impervious barrier paint.

Safe to say I won't be getting this from B&Q, I should expect to pay around 1/2 their price.