Will this work..or is it too big?

I've decided to build an airboat in my spare time. I plan on building it out of 9mm plywood all round, and powering it with an OS 25 LA engine. If I stuck to these plans, would it A)work well, and B)be powered well enough by the 25 LA to get some decent speeds out of it when using a 9x6 propeller?

At the moment I'm wondering if 5" is too much for the height, and maybe I'd be better off building it a bit lower?
Also, is 9mm a bit drastic - would 3mm or 6mm plywood be best?

Thanks,
Marc.