If you plan on SCUBA diving in the Galapagos and have your own wetsuit, bring it. You will need at least a 6mm wetsuit, boots, gloves and possibly a hood. While most dive trips have wetsuits to rent, but your own will be the best fit.
In addition, dive trips usually have a regulator, BCD, computer, weight belt, fins and mask to rent. Some of the dive trips will include equipment in the price of their packages, be sure to check with your specific trip for more details.