Howdy Mamaw! I've always had good luck with You're the Captain boat rentals. They're located at Hamilton's Restaurant at Thomas Drive. Spoke with Mark over there to get a price breakdown on their pontoons -

16ft for up to 6 people runs $109/half day or $179/full day.
20ft for up to 9 people are $129/half day or $199/full day.
24ft for up to 12 folks are $159/half day or $219/full day.

These pontoon boat rentals are a blast Mawmaw, and a pretty good value. Everything you need for boating is provided - life jackets, gas, etc. - except your personal stuff. They'll give you a run-down of how to operate the boat, and a map to help you get around.

And since this is the off-season, I'd ask about specials, etc. You just might get a better deal. Good luck and have fun!