welcome

by the way, i was there this past September. The guide said that EL Nina was in affect and the water was "warmer". Sorry, i didnt bring a thermometer or was able to measure the temperature to know what it was and or do it before or after El nina to measure any difference.

you may want consider ACCLIMATION.

this is extremely relative to each and every person in my view/experience.

Here is the one i like to use since it has the info (monthly averages) for the whole year. Im not sure if you go/use this, but its what i use. I dont use it to "predict" so to speak, but to get an idea on how to pack/dress.

https://www.holiday-weather.com/galapagos_islands/averages/

Note. it was my 1st time snorkeling and i used the above site to see how "warm" it is - compared to what im ACCLIMATED to. if you want, look at the water temperatures where i live.

in any case, i opted to rent the wet suites. They had the 3 mm version.

also, this is how our ship did things...not sure how others will ...

> the first time was to get everyone outfitted with wetsuits/masks..ect on the ship and then take a dingy to a protected/shallow bay/cove to see each persons level of comfort. Some people didnt swim and also used vests, like me. Some people brought their own mask/snorkels/suits too.

> then we spent that time either snorkeling in that area or explore the small island. some also picked up trash too.

> in any case, what i learned/found out and others doing that, was that the water was "warm" to me and others. Noting ACCLIMATION and what im comfortable with. So, on the other snorkels i just used the life vest and that was warm enough - for me. Again, acclimation.

> note there was a girl from Hawaii that had alot of snorkeling/swimming time and all she wore was a 1 piece swim suit. but she was cold in/out of the water unless the sun was out. Again, acclimation.

If you know what ship you will use, you can ask them what they may offer and rent/use if you want. Or if you want and have the space, i would bring the 5mm suit. why? because you say ".. I get cold easily..." to me, this says it all. If you are like anyone else i met like this, you wont be a happy camper. My mom was like this so i know.

good luck