Not saying that brushless won't work in a boat, just not as simple as throwing any motor in there. A rigger might be more receptive to a high-kv motor than a v-hull, but 8000kv is just too much for direct drive in any boat. Take a moment to consider that just getting a lower speed motor gives you two motors and you can swap the controller between boat and car (as I do) for double the fun!
For the price of a good gear reduction unit you could have this perfect boat setup:

http://cgi.ebay.com/ws/eBayISAPI.dll...976957476&rd=1

(tempted to buy it myself)
Edit: That shaft size is exactly right for any speed 400 motor coupling.