Steerable drives aren't very efficient in any model size, and have limited steering range. They could be nice on scale models, but I don't think you'll get good performance for a quicker boat.
A foam plate is much thicker than 1 or 2mm, and a plastic sheet the same stiffness would be much stiffer (and heavier). I'm not saying foam won't be stiff enough, just that it won't be stiffer than the plastic.
One challenge with forming foam is that you need two parts of the mould (male and female) to get a nice part. Because of this, you could get REALLY accurate/sharp running surfaces which is a big bonus.
I also think with some thin bulkheads you could bring the stiffness up without increasing weight much at all.
This link might help too: