@ToddL. I think your estimate is a good start. Of course, lots of other costs could be added. Tournament fees. Travel and lodging to tournaments. Here in CA, I see most of the Top kids take far more lessons than what you have estimated and the cost per lesson is often higher than your estimate. Seems like the top kids also go to Florida, Louisiana, or elsewhere for training. Gloves, ropes, handles, vests, etc. add up as well.
I think a three event might be a better comparison to snow skiers as snow skiers from 10-15 usually ski SL, GS and Super G, and have 3 to six sets of skis, 3 sets of poles, and the specialty shin guards, arm guards, back protection, helmets, etc. for the disciplines. Similarly in water skiing you could count the trick skis, jump skis, trick ropes, toe ropes, jump suits, helmets, arm slings, etc.).
I would think that without being too ridiculous in the estimates, the junior water skier number could go over $200K.
Seems as though it's costly to raise a top athlete in most sports these days, and particularly in equipment intensive sports.