"Dad and mom want hot water."
In 热水, 热 ("hot") describes 水 ("water") to mean "hot water."
In 水的热, 水的 ("of water"/"water's") describes 热 (here being the noun "heat" instead of the adjective "hot"). Like 16kacit said, 水的热 would mean "water's heat" or "the heat of water."
That said, 热的水 would be a valid answer. Adding the 的 here doesn't change that 热 describes 水. It just specifies "water that is hot" or "THE water that is hot." It's not necessary, though you may say it like this to emphasize hot water over another type of water.
Hope this helps!
This whole lesson is full of awkward Chinese phrases. One should say "我要喝茶“ not just "我要茶.” The verb for the eat or drink is important contextually and also to balance the two word preference in Chinese. My Taiwanese wife would look at me in an upset manner if I asked her "你要不要茶？“ and left out "喝“