Right. If you run the stock motors at 12 volts there is a good chance the motors will overheat and hence have a short life span.
The supplied motors (Speed 500) should have more than adequate power to submerge the boat at 6 volts.
In fact they tend to be a little juicy, so a lot of folk have replaced the stock motors with Speed 400's reduced with a toothed belt drive. These are sold ready to fit by one of the sub vendors, I think it's 'Ships and things', but I could be mistaken.
What grease are you using in the shafts?
Packing the shafts fully with grease will load the motor considerably.
I'm not that familiar with the Robbe kit, but I'm sure the shafts must have some sort of rubber seal/stuffing box to prevent water ingress.
If not consider uprading the shafts, Subtech do a good range of sealing devices.
Also use a good quality electronic speed control with your sub. A switch is bad way of controlling a model sub, crude and too many nasty things can happen to a switch which could easily result in the loss of your boat (i.e. it gets stuck on).