My original seat base was naked as a jaybird, turned grey with age, splintered but not rotten. It was 9mm SEVEN ply plywood.
I gave up with the 4/5 ply C__P flogged by local woodyards and finally tracked down some real mcoy at a Bristol marine supplier: Robbins Super Elite. Its the best available,BS1088 RNLI/LLOYDS approved 9mm thick marine (7)ply mahogany for persistently wet/dry areas and is very expensive, £37.18 +VAT+ p&p.Minimum amount 1/4 sheet.
I have two pieces left over each ample to make a seat base. Anyone interested at £18 a go plus p&p give me a call on 01772 497505 or PM me, saves forking out for a 1/4 sheet and having loads left over, Bob