I see, try 13t, since the brushless Slash 4x4 comes with it. And yes, the program box you linked will work, it's the same model that I have. Just to clarify though, programming the ESC does not require any external programmer, but having the program box makes things a lot easier. Here's the...
I have never run my RCs in water, but they can generally take a few splashes here and there. Submerging them for a couple of seconds probably won't hurt either, but any longer may risk damaging the ESC. I have seen people coat the seams of the ESC case with shoe goo, or elastic glue to seal it...
The Max10 SCT with the 3660 size 3200kv motor: https://www.amainhobbies.com/hobbywing-ezrun-max10-sct-sensorless-brushless-esc-3660sl-motor-combo-3200kv-hwa38010200/p-quartzrq7mzxactz
The 4000kv cannot take more than 2s, and the 3200kv runs cooler.
The motor combo linked here has a 60 amp ESC...