Hi @JF2021, welcome to the forum 
Max speed depends a lot on the mass being accelerated, and the surface and drag area of the boat. As a very rough reference, this post discusses using two T200 thrusters to propel a human diver underwater (using our 4S6P battery), and ends up with an estimated max speed of about 43m/min, which is ~1.6mph.
Presumably your boat will have a decent amount less drag than a human diver, since it’s smoother, partially out of the water, smaller, and doesn’t need oxygen tubes and the like. I would also guess a remote control boat of this size would be lighter than a human (and perhaps quite significantly so), but that depends what you put in it and make it out of.
Without knowing more about the boat properties it’s difficult to give a meaningful max speed estimate. My intuition tells me you’re unlikely to get 10mph with a single T200, but may do with two or three. Then again, I don’t have much experience with boats so I have very little trust in my intuition here, particularly given the lack of important parameter details
As per the T200 Product Page they’re rated to use 7-20V, and we don’t recommend going above that without power limiting, so if you want to use 6S you’ll need to be quite careful with how you set up your throttle control.