I have faced this several times myself, and I would second what spikesmom said about skipping the water park, but they are obviously very popular, especially with teens.
Devote one full day to Magic Kingdom
Animal Kingdom is a half day park at a full day price, but be sure to go in the morning, when temparatures are cooler and the animals are more active. That would be a good park to combine with your half day at the water park
Others will not agree here, but with most teenagers, half a day can be devoted to EPCOT (Center) when you have limited time.
Meals or wheels will be the key in deciding whether to hit DisMGM early and EPCOT (Center) late, or vice versa. You couldn't really go wrong going to DisMGM in the morning and coming over in the afternoon to EPCOT (Center), wrapping up the day with a viewing of Illuminations, but I would do just the opposite only because I prefer to wrap up with Fantasmic. The evening dining is much better at EPCOT (Center) though, for sure!
P.S. - Are your teens going to stay with you throughout the day? If so, awards and accolades to you! My wife and I get to see our teenager when we arrive together and go on one major ride together, then only at major meals and when we are leaving together!

:lol: