Hydro Junkie

I cannot answer on the drive questions but I can help with the construction ones:
For the deck im thinking 3.6mm ply This would be fine since you're not going to be dealing with major or compound curves while at the same time, it's strong enough to handle the load of any superstructure or detail parts sitting on it
and 12mm ply for the bulk heads, would this be ideal? NO, WAY TO HEAVY!!!!!!!!!!!!!!!!! I'm assuming it's a fiberglass hull so frames as thick as what you're thinking about is serious overkill. I build 1/8 scale hydroplanes and don't exceed 6mm ply for any part of the build, that being used for the boat's transom and the right side transom. The only reason I use this heavy of material in those two locations is that the rudder and turn fin are mounted to these frames. The rest of the boat is built using 1.5mm with 3mm used where more rigidity is needed. Since my boats are roughly 1.1-1.2 meters long, .5 meter wide and run at up to 100KPH, they are subjected to considerably more stress and pounding while running than your tug will be. All that said, I would look at the weight of the drive batteries. You might need the 12mm to support the weight due to the ply needing to lay flat instead of on edge
Also what thickness ply would be nest to use for the superstructure? I would use 3mm for any part that will be load bearing or butt glued at the end. Another option would be use use 3mm stick stock as gussets and use 1.5mm for the bulkheads. Everything else I would use 1.5mm to save weight. The last thing you want is a top-heavy tug so anything you can do to reduce topweight will aid in keeping the center of gravity lower