Depending on the number in your group and the length of your visit, you might want to consider doing the family Length-of-Stay recreation package, which allows up to 5 people in your group to have unlimited watercraft and bicycle rentals. Last summer, my family used it and REALLY got great value from it. It was $199 + tax (maybe $210 altogether), and we did it for 3 adults and 2 teenage boys. The boys used several hours worth of water mice, as did we adults, plus DH & I took out a pontoon boat for an hour and we adults also took a spin on bikes over to FW for about 3 hours one day.
It is good for your length of stay, in our case 8 days. The boys were not allowed to take a pontoon boat, but they were happy with the mice. Depending upon how busy your park touring schedule is, this is a wonderful option. We liked the WL marina location since you can ride through the connecting lake over by MK, GF and the Poly.
Lots of people on these boards are aware of this package, but I didn't see it posted on the rates board at the rental office. They knew just what I was asking for, though. Minors must have a parent sign a waiver, but if you have teens they don't have to have you with them after the initial sign-up.
Let me know if you have any questions about this - and have a GREAT time!!!