You will receive a passport stamp, which can be a problem if you're not a US resident/citizen and come from a visa-waiver country, as you will no longer be eligible for ESTA to travel to the US, having visited Iran.

Snorkeling is one of the biggest draws of the island — it's some of the best in the world. There are many hotels and it's probably easiest to just find something upon arrival, but there are Tehran-based agencies that can take international payment methods if you prefer to pre-book.

The island is dry, as with the rest of Iran. It's a very small island and I'd suggest a pre-organized tour may not be needed — asking around upon arrival should suffice.

If you're female, there are a few additional considerations: you must cover up, but they actually provide clothing at the airport which you can return upon departure. Also, the best snorkeling is from beaches that are off-limits to women.