Welcome
weather and temperatures are relative and how acclimated the person is and how tolerant they are. Its relative to the individual.
here is a site i use for temperatures.... its an yearly average based monthly.
https://www.holiday-weather.com/galapagos_islands/averages/
note i was there 2 years ago in a September and el Nina? was suppose to be going on. It was suppose to be "warmer" too. I tried my wetsuit and after getting into the water, i decide to not use and just use the life vest. I found the life vest kept my upper body warm enough and we were not in the water for hours either.
To me and where i come from, the water was "warm" enough. But i will say one of the other passengers on our ship was from Hawaii and she was "cold". There was a mix of people using and not using the wetsuits.
Our ship offered to rent wetsuits, but it was either or, not cherry picking when.
on the islands i did see shops that offered to rent all of the stuff and including gopros. On the beachs i visited, i didnt see anyone wearing a wet suit. But its up to you.
good luck