Since the setup was probably bought that way, I would give it a try in the tub, the pool, or at the bank of the pond. The set up needs to be tested under load, pushing water.

Hold on to the boat, and run it up. After a few minutes, if the wiring only gets warm you should be ok.

If the wiring and gets hot, you might consider buying updated electronic speed controls.
Smallest hazard, the wiring gets hot enough to desolder any connections... Seen that happen. Worst hazard is that the wiring over heats burns the insulation and possibly some of the components or the hull.

Edit]
See how "Electricalgod" has used three blue Proboat style Electronic Speed controls here?
http://www.rcuniverse.com/forum/m_6085599/tm.htm
One for each motor. However his motors may draw more than yours.
Testing for checking for heat or the amp draw using a multimeter the best way to verify your set up

Without placing a multimeter on the motors and testing the amp draw, it is difficult to give you any better advice or recommendations.[]