Thank you! That is very helpful information. I did find one that guarantees return to the ship in time, but it's a large group, and I was hoping for a smaller one.
I figured if we leave by 8:30, an 8-hour tour would get us back by 4:30. But looks like that would be too late. There is one 7-hour tour that looks good, but it doesn't start until 9:20, so still too long. Interestingly, the one that guarantees return (geared for cruisers) leaves earlier, at 8 or so I think. So that's how they do it.
Will keep looking!