Well, I already owned all my gear. I wouldn't run out and buy a 5mil suit just for snorkeling. If the boat offers full suits, those will be adequate. If they only offer shorties, you might be able to luck out on a deal on a full suit at a local dive shop (your hometown, that is); they often offer their used gear at killer deals.
One bit of advice for when you get out of the water: remove the wetsuit immediately, and dry off. The sooner your skin is dry, the sooner you'll start to warm up.
Edited: 27 May 2022, 01:25