Hi and welcome to the forum,
It sounds like the same problem that many of us have had, which is that the carbs need to be completely stripped and cleaned as some of the tiny passageways can get blocked with modern fuel waxing. If the fuel has been left in the carbs rather than the carb float bowls be drained off after use.
If you can get the boat on the water then when the problem starts then try blipping the choke on and off a few times and see if that gets you through the midrange problem. It should do and will confirm that it`s a blockage that`s causing the problem. I know it`s not the most technical way of testing it but i find that it works for a quick check.
Matt