I have also used Captain Marvins fir Stingray City. We went on the last tour of the day. We were the only boat out there. The cruisers had already returned to the ship. It was amazing.
Yes, we were the 1st tour of the day and went on a day when there were basically no cruise ships. Similar experience - only ones for most of the time we were there
Days toward the middle of the week are the heavier cruise days given Cayman is further south. Tends to be a mid week stop for Sun-Sat cruises.
Planning your visit to Stingray City on days when there are fewer cruise ships can indeed provide a more peaceful and less crowded experience. It's helpful to know that mid-week days tend to have a higher number of cruise ship visits.