Gotta be a little faster than that if you are pulling on Jet Skis… Mine claimed 58mph on the speedo when I had it…
I’d be very wary about building a boat from scratch. There are considerations with regards to venting the gas fumes, making sure you have flame arrestors on the carbs, cooling, making sure no oil or fuel can get into the water, etc. If you do it, you really need to make sure and do your homework.