It is fine, and on a clear day genuinely nice. Just book the trip for the scenery and the full day out, with snorkelling as one part of it.
A basic mask and snorkel comes with most tours. If you are fussy about fit, and plenty of people are, bring your own mask.
Yes, and many do. The boat, the beaches and the viewpoints are the bulk of the day. Rules for individual activities vary, so tell the operator when you book.
We would rather tell you this now than have you disappointed on the boat. Snorkelling here is a nice extra. It is not the reason to come. Ang Thong earns its reputation on limestone cliffs, kayaking, beaches and viewpoints.
Three to five metres is a fair expectation. Wind, rain, tide and the site the crew picks all move that number. Some days you get more. Some days you get murk.
Storms, warming water and decades of boat traffic have taken a toll on parts of the reef. There are small fish and a pleasant swim. There is not untouched coral, and anyone promising you that has not been recently.
Wear the life jacket when you are told to. Stay in the area the guide marks out. Never stand on coral, even when it looks dead. And say something to the crew if you are a weak swimmer, before you get in.
Do not pay a premium for the word snorkelling on a listing. Look at the whole day instead, on the comparison page.