check out my thread:
Super Student Shakespeare Shenanigans
its a bit long but in there are all the detail of how I did it. You can use WBP ply which I was told was the same as the marine ply but without continuous veneers, which doesn't matter for a jockey seat. WBP is cheap as chips, perhaps about £10-£15 for an 8'x4' 5.5mm panel.
I used 5.5mm ply with a couple of layers of 300gsm GRP on the outside for non structural bits, but used 9mm ply on the seat sections. I also used 9mm ply for the rear panel where my backrest bolts on + 9mm ply backing plates...perhaps overkill but safe as houses.
I used epoxy to coat the inside as it provides a hard wearing finish. G4 just kind of soaks in I think so not sure how good it will be. Use G4 to coat the surface before applying your GRP layers if using polyester resin.
Hope that helps.