We went last spring. We couldn't get in at Bahia Honda so we stayed at the KOA on Sugarloaf Key. It had a nice bath house, laundry, water and electric hookups, bar, restaurant and a small fishing supply shop. It also had a small beach, boat rental and a kayak launching point.
The state park is nice and you can snorkel right from the beach. It was a little windy when we kayaked there and they have a nice parking area and kayak launch area so if you can get a relatively calm day it will be really nice. We kayaked from the KOA at night after returning from the park and it was really nice with low wind. The beach was covered with a 12" tall x 24" wide sea grass mound at the waterline and behind that was a coral surface that was hard, so I recommend chairs. If you are lucky you can find the few sandy spots but they get taken up pretty quick.
John Pennekamp has a nice kayaking trail that I really recommend. We also did the snorkeling trip from there but when we go back we plan on doing the trip from Bahia Honda. I think they were both $35 plus rental of gear. Heck we may do both.
One thing that was crazy was when we were setting up our screen room we had 6 iguanas come up to us (at one time) looking for handouts like raccoons. We ran them off but be aware that they seam like the local scavengers. One was really cool and over 4 feet long.
"Twenty years form now you will be more disappointed by the things that you didn't do than by the ones you did. So throw off the bowlines... Explore. Dream. Discover" Mark Twain
John & Tatanya