We hired a private charter in Grand Cayman for our family of four. It cost about $350. Another man, not the company owner, took us on a small boat to Stingray City, snorkeling in coral gardens and to starfish beach. Tour was suppose to be four hours long but we cut it short after three hours, our choice. It was awesome all the way around but one of our daughters was DONE! We tipped him $40. I am normally a generous tipper (former waitress) but, for some reason, tipping didn’t enter my mind until we were already on our excursion and $40 cash is all we had with us.
There was a recent thread about excursion tipping. I would encourage you to look for it as there were some very detailed responses.