While snorkeling in the Galapagos, wetsuits are especially recommended from April to November, as this is the season when currents make the water cooler. Even during this time of year it is still possible to go in without one, although you may not want to stay in the water long. Even during the rest of the year, having a wetsuit might make you more comfortable in the water.
Most yachts provide wetsuits for their passengers, either for free or for rent. Even if your yacht doesn’t provide wetsuits you may be able to rent them from a dive shop in the Galapagos Islands.
The wetsuits used for snorkeling are usually 3mm shorties. Divers generally use 6-7 mm wetsuits during the cold season.
Some boats provide snorkeling gear, some rent and some may not have it at all. If you have your own gear, go ahead and bring it, as it is most convenient and more sanitary. It could also be possible to rent gear in Quito as well. Check with your specific trip for the details.