HI Les
There is no problem using two 7.2v batteries in parallel and the 9000 ma will Improve your performance without damaging the ESC. The extra power from two batteries is due to the load being shared and they are able to better support the lower individual drain. You will need to check that one cell does not discharge quicker than the rest as this may damage the battery if its voltage drops below 1v. Not easy to check in a sealed battery but any extra hot cells may be an early indication.
Dave

Hi
Think you have made the right choice better safe than sorry you can always upgrade to lipos on your next model LOL
Safe boating
Pete

Thanks for the advice.
I wont risk it.
I was running the boat on two 7.2 volt Nimh in series and this is what prompted me to wonder about using the Lipo.
However I did find that compared to the old brushed motor, the brushless motor makes quite an Improvement on only one 7.2 volt Nimh battery. I have a pair of 4500 mah Nimh and I thought why not put them in parallel to get 9000 mah at 7.2 volts.
With this configuration there was a noticeable step up in performance over a single battery which was a bit of a surprise because I was only expecting longer running time. I thought theoretically its 7.2 volts on one battery or two in parallel but there is a noticeable difference on two.
Anyway I'm a happy boy now.

Les

I second that advice. The ESC will have components inside that will explode if you use too high a voltage. To allow for tolerances there will be a measure of over protection, but this is a safety net and should not be used continually.
Glad you are finding LiPos so much better than NiMh. I did warn you you would want to change all your models
Good luck
Dave

My personal advise DONT DO IT Can escalate into a fire in seconds if you want to upgrade to 4s lipos it's a lot cheaper in the long run to purchase a new ESC say about 120A than to buy new batteries,recever,motor and possibly a new boat to put them in.
Good luck
Pete