For sensored brushless racing in the wet you need a motor without any cooling holes. Current ones on the market are the Tekin redline, Novak ballistic and Losi Xcelorin. You then need a tub of Plasti dip to cover the joins in the motor, plus the sensor plug, plus all of the case joins in the ESC, around all of the ESC wires etc.
If everyone gets the same kit then the Mtroniks is a cheap, waterproof option:
It won't be very fast, and it may cog a bit at low speed, but if its the same for everyone it doesn't really matter.