We used them last March in Grand Cayman for the stingray excursion (we were on the Hannibal).
We thought they delivered exactly what they advertised and no more. I missed the fun and personality of some of the other tours that we've experienced. It was not unpleasant but definitely "just there".
We're going back to Grand Cayman next March and will likely do the same excursion (we're bringing friends that have never done this before). We will not, however, use Island Marketing. Not sure who we will use yet but I've heard positive reviews from others about other places so I'll do some research.
That's just my opinion, so others may feel different but...