I doubt the fan will help much. The volume of air inside the boat will be a few liters at most, and the fan will only transfer the motor heat to this air. If you have openings for air from the outside to enter and exit, it will cool better - but then, water can get in, too.
Regarding "Artic Ice", if you meant Arctic Silver then yes - same kind of stuff. Radio Shack lists it for $6.99.
If you are really curious, you can use a temperature logging device, like from Eagle Tree or similar. I have inputs for battery and motor temperature on my Schulze ESC but have not hooked them up yet.
You can use an infrared thermometer to check the temperature after the run, but of course logging under load is the best possible metric.
The best way to keep your motor from overheating, though, is not to put too great a load on it - that means a reasonable propeller size and pitch.
If it is a low-resistance motor like these little outrunners, though, you will create some heat anyway.
I found my motor generated a lot of heat without the cooling, but this was with a 29mm diameter / 29mm pitch prop, and a little less with a 27mm diameter / 34mm pitch one.
However - that boat has a submerged prop! This "Mini Mono" is surface drive.
I was emptying 2200mAh 45C packs in about 4-5 minutes.
I would be interested to know how this boat runs on INrunner motors. I have found and heard as well that the torque from the bell of outrunners can cause stability issues - if and when the boat rolls sideways off a wave, the momentum of the bell tends to keep it rolling!