The best thing to protect the motors is to put them in a splash proof housing. This will ruin your motor cooling which can burn up your motors depening on how hard you push them. You could try to find a way to keep out the water but still let in some air for cooling.
One way to avoid motors melting is using sensored motors that have a temp sensor, and use a esc that will softly cut off if the motors get too hot (the vesc can do this). The problem with sensored motors is that the hall sensor pcbs are probably not waterproof so at that point you absolutely have to keep the water out.
Using chunky aluminum motor mounts and applying thermal paste between the motor and the mount will really help with the cooling!
You will likely need fenders to keep dirty water from spraying onto you from the wheels
You will likely need wheels that have grip in wet conditions (pneumatics are probably much better than any urethane wheel)
Get a chain drive, not a belt drive, be prepared to oil your chain every other week when its raining
Good luck i will follow your build as I will also attempt to make a waterproof board when I get back to the Netherlands