The Graupner speed 600 7.2V motor at the bottom of this page has 20K rpm and would be good if you are running one 6 or 7 cell pack in your boat. If you are running two six battery packs then go for the 12V motor near the bottom of the page. I would recommend Offshore but he is out of the 600 race motors.

http://www.hobby-lobby.com/speed600.htm