On our 1st 2 cruises, we didn't.
Our 1st one was a very large group that was simply taken to the beach.
The 2nd was a catamaran sail. Again, there were quite a few people on this and nothing special was really done. I didn't notice anyone else tipping.
On the last one (St. Thomas Capt. Nautica), this was a small group, the guides were awesome and informative. They did mention that if we would like to tip it would be appreciated, but it was no big deal if we didn't. I was just glad we had brought some cash because they really deserved it.