I’ve been to the Galápagos three times (naturalist cruises and a live aboard dive boat). Select your cruise carefully. (The first cruise we were due to be on, the boat caught fire at sea - that got us to be more cautious, though fortunately the fire occurred months before we were due to board. The second boat was okay, but it brilliant - air conditioners were normal Land window hung devices which, though new, were already rusting and preparing for death.)
There is no safety-tasked organization, like Transport Canada or US Coast Guard, vetting the boats.
Be aware sime cruises are scams - a seven day cruise can be three and four day cruises sold together - you are visiting Islands close to your port of embarkation and wasting a day at that port in the middle.
Be aware different sized ships may be destination-limited. Some Islands are limited to small groups - the larger boats can’t stop there.
Be aware some boats have basic naturalist guides, others have u ivetsity trained naturalist guides. Your experience will be far better with the latter.
My advice is go for a cruise with sixteen or fewer passengers, and one sufficiently long you will see some of the westernmost Islands.
Unfortunately, Barry Boyce stopped publishing his excellent Guide to the Galápagos; he rated boars and agencies, and offered a huge resource to people interested in this unique, normally once in a lifetime, experience.
But do some preparation. Bradt, Moon, Lonely Planet, etc. do publish guides, and a natural history guide (Fitter and Fitter, Jackson, etc.) is indispensable to get the most of your fauna encounters.