First, depending where you've snorkeled in Hawaii, NOTHING is like snorkeling there. Just know that now and don't expect to be able to compare it. (We have snorkeled off Oahu, Maui, Molokai and the Kona coast and no place compares.)
We did the Chileno Bay excursion back in (July) '05 and the water in Cabo was FREEZING cold. It was so cold I couldn't breathe. We jumped off the boat & swam to the closest ladder & got back on. I felt like I just jumped off of the Titanic. DS however did not follow us and swam straight to the shore!

DH had to go back in and swim over to the shore to get him and bring him back. We had jellyfish in Mazatlan not Cabo. Cabo had sea lice (apparently brought in with a recent storm).

We spent the entire time on the boat. It was a beautiful day and a beautiful trip and I got a lot of great scenic pics, but the water was NOT warm enough to swim in without a wetsuit.
The water conditions in the summer in this entire region are going to vary greatly depending on any storms which may have come thru recently. Summer down there is the rainy season (i.e. hurricane season). That is why we're not going back again during the summer. We would like to do the Mexican Riviera again, but during a different time of year. That's why we decided to do back-to-backs on the Wonder this summer instead.
Hope you have better luck than we did. Just because we struck out does not mean you will. I hate to tell you this, but I think your individual experience will be determined by the weather immediately preceeding your trip. That's what will determine the water conditions for you.