Hi Greg,
The Cen Aqua Jet was originally sold in two versions; one with a docile 600 motor 8,4V set-up and one with the 700 motor on 12- 14 cells NiMH.
The 700 motor version is the most exiting to run, but runtimes on 3300mAh NiMH were 8-10 minutes full throttle, depending on the propchoice.
The 'average' 700 motor used, turned about 1600 rpm/Volt, with brushless motors it's called KV, so to have a fitting replacement, look for a brushless motor with 1600 KV, which on a 4S lipo gives you about 21.000 rpm under load.
It doesn't necessarily have to be an inrunner, as an outrunner has more torque and in general is lighter.
As an example, my 65 cm long monohull runs over 30 mph on a 1700KV brushless outrunner on a 3S (11,1V) Lipo, which is close to the speeds this boat used to run on the much heavier combo of an 700 Neodym and 12 cells NiMH.
It's the white boat in the video:
http://www.youtube.com/watch?v=I2h1kWwaFCAThe conditions were rough, but you get an idea...
Runtimes are about 10 minutes, depending on propchoice and battery capacity.
Let me know if I have to get you some links on motor, ESC and Lipo for such a combo.
Regards, Jan.