"What tour companies offer it ?"
Google N Resort Jamaica.

"Where do they fly out of ?"
Any airport that has flights to Jamaica. I don't believe the charter co's that offer N have direct flights to Montego Bay. We used ATA thru Atlanta.

"What cost benefit is it for booking thru a tour operator?"
You'll have to compare prices yourself.

"What is the beach like for walking nude ?"
Facing the ocean, you can walk for 2 miles as nature intended without seeing another person or civilization.

"What snorkelling is there ?"
There is a reef of shore that is ok. (Provided the water is calm enough}