I am very brutal with my stuff, and have submerged an Arrma Granite with both a Castle motor, and a DHK Hobby Wolf 2, with various BL motors (it currently has a GoolRC 3660 3800KV in it) in water almost above the body, and, so far, in 3 years of running stuff that way, have destroyed 2 motors from water (both were cheap, supposedly waterproof stuff i got from AliExpress). But, i also take my stuff apart after running it like that, clean everything, and dry it out. I used to not do that, and completely ruined a couple different vehicles. My favorite time to run is in the spring when the snow is melted but not all gone, because the snow makes awesome jumps, especially if there is a puddle to land in. The Granite and Redcat Blackout make the best cannon ball splashes, because of the weight. One thing doing that taught me is what works, and what doesn't.
One thing i dont think was mentioned. Make sure to check for motor shaft size. I learned the hard way, especially with the cheaper motors, some come with a 3MM shaft, and some have a 5MM shaft. For the Slash, with stock gearing, i think you need a motor with a 3MM shaft.