Marine ply is great stuff for big boats, but not much use for models. It's all stamped BS1088, they don't make it any thinner than 6mm, or occasionally 5mm, it's heavy as anything, expensive, and it blunts your tools as it's b****y hard to cut. All we need is good quality ply, preferable birch. The stuff to look for is stamped WBP which means 'water and boil proof'. That means that the adhesive is 100% waterproof, at least as far as modellers are concerned. Places like B & Q sell cheap 3mm & 4mm ply for flooring purposes, it may seem like an economy, but the wood is poor quality and the adhesive isn't waterproof, so it will cause problems eventually. Many of the original Aerokits kits were made from ply like this, and most of the surviving examples are suffering from delaminating bulkheads now. Spending a bit extra on materials like plywood makes construction a lot easier as well, even an old skinflint like Bluebird avoids the very cheapest stuff.