I doubt if a cooling fan would be sufficient, not unless it moved a fair quantity of air, but in doing so, its going to dramatically drain the battery power and reduce sailing time by a major factor.
using the water the boat is passing through is far more efficient and does not drain battery power.
Plus it would then mean having to have open spaces in the upper boat to allow passage of the airflow, not recommended with risk of sinking if anything happened on a tight turn.
my Pt 596 can swamp itself on 3 x graupner 400's on a very hard turn, so know only too well what can happen, so a brushless setup could be very quick, even on just a single motor, providing you keep the weight to a ultimate minimum, which negates the use of wood under the motor, you're putting a lot of unwanted weight in, which then means bigger motor, bigger esc, bigger battery.
Dont forget about the esc too, you need to fit a cooling coil to its underside, not unless you have a proper "brushless boat esc" which has the water cooling jacket already attached, so all you need to do is plumb it in.
you can buy a water cooled motor mount if the motor is an "outrunner" version, or if its an "inrunner", you simply slide a cooling jacket over the outer shell, both items are not a lot of money either.