If you look at the kit on the SLEC web-site it tells you what you need! They even sell the motor and ESC.
If you don't want to use their makes you could look for similar spec. motors from Turnigy, Surpass and Overlander.
An alternative ESC is the HobbyWing Quicrun 30amp. It's not water-cooled but you shouldn't really need it with that motor and ESC as they shouldn't be working very hard on a boat of that size and weight.