I suggest a bit of on line research. This is a 21 inch long launch, any number of them feature on model boat threads. I picked up a lot of useful information reading such threads.
At 1:15, 25kt full size gives nearer 7.5 mph scale. Divide the speed by the square root of the scale, not the scale.
I've had a Graupner Lotse (similar size & performance) doing long, fast runs using a 7 cell NiMH pack driving a Speed 400 motor for that voltage. I can cruise it round my lake, a 1 mile walk, without any problems. In practical terms, when the battery gets low, it's time to go home anyway. Using modern technology might improve it, but I can't be bothered.
Try not to over-think problems. Usually, they just get bigger, don't get solved and the thinking becomes a displacement activity that puts off the build.
Have a look here http://eezebilt.tk/index.html for a simple low cost learning experience that will answer many of your questions regarding the general principles of model boat making.