Those tamiya esc's hate water as do most simple brushed esc's. electric motors aren't too keen on water when they're running either.

this might be a solution(as kids will always run till they bust-just as we did and still do today)

http://www.ebay.com.au/itm/HPI-RACIN...item19cc2e5ab6

the hpi esc are good units and this one allows a motor upgrade when you're both ready.