Well, turbines aren't 100% efficient, so trying to gain power from charging via an underwater turbine while powering your motors via solar, won't gain you power, but will instead lose it. Your forward motion costs say, 4kw per hour. The hydroturbine setup would then cause power losses when generating power, as every joule that powered it would come from motion caused by the motors moving you forward (which also aren't perfectly efficient), and then the turbine is say 90% efficient, meaning that even at that awesome efficiency rating, you're losing 10% of the power you're using to generate the power in the turbine - and that ignores drag on the hull, and other losses such as the energy expended in moving the inertial mass.

There's no perpetual energy motion machine, and all equipment experiences losses. This is why superconduction is so sexeh, because superconduction is lossless.

In a current while at rest, however, such a rig might well produce significant power.