Egmont is worth $10, regardless of the snorkeling. The island is uninhabited now (except for a few pilot boat captains), but there used to be a small town on the north end of the island. A lot of the foundations and parts of buidlings are still around. The best part is an old fort (WW1, I think). You can spend several hours walking around the old roads and paths. Plus, the beach is one of the best in the US. Beautiful sand - similar to DeSoto and Caladesi.
Regarding the ruins: The island used to a hundred yards further west (into the Gulf), and there were buildings on that part of the island. Due to hurricanes, dredging, whatever, that part of the island is now under ~10ft of water. The water is normally very clear, and there are a few fish, but nothing remarkable. There is no bottom structure, like plants or coral. A lot of folks like to jump off the building into the water.
I personally wouldn't do the snorkeling this weekend because the water temp is still around 74-75F, and I don't own a dry suit. ;-)
TT