Snorkelling on a Pig Island day happens at Koh Tan, not at the pigs' beach. The boat anchors over a shallow reef on the side facing Samui and you go in off the stern with a mask, snorkel and a life vest. It is a beginner reef: two to five metres deep, small reef fish, patches of hard coral, the odd sea cucumber and, if you are lucky, a turtle.
Forty five minutes to an hour on the shared tours. As long as you like within the booked hours on a private boat.
Mask, snorkel and vest are on every boat. Fins are not standard. Bring your own mask if you have one that fits well; hire masks leak on people with beards.
Nobody has to go in. Guests who stay on deck at Koh Tan still get the pigs, the meal and the ride. The catamaran is the boat to choose if you want a slide and paddle boards as an alternative to swimming out.
Do not stand on coral, do not touch anything, and wear reef-safe sunscreen or a rash vest. The reef is shallow enough that a careless fin kick does real damage.
You swim in the shallows where the pigs wade, but the reef and the fish are at Koh Tan. The two islands are fifteen minutes apart.
A little on the rocky ends. The snorkel stop is deliberately at Koh Tan where the reef is better.