Using a speed control with more cells than sugested is not a good idea.
Most of the speed controls made today will tolerate a slightly higher voltage input of a cell or 2, but you take your chances. Some of the components of the speed control can be quite voltage sensative depending on the brand you choose, and an extra cell could just send them over the edge.

Current is going to be you determining factor.
Since you give no description of the application other than the turns of the motors, I will assume a high current application to be on the safe side.

The need for reverse is going to be a limiting factor. None of the good high end speed controls for boats have reverse. There is no need for it in a high current racing application.

If you are building a slower scale or fun boat, there are some low end speed controls out there that have reverse and will handle higher cell counts.

Give us more information, and I will see if we can come up with a list for you.

Peter

www.climatemodels.com