A second determination factor might be for you to decide what level of adventure you would like your snorkel tour to achieve. If you are a high-octane adventure enthusiast you might be more inclined to enjoy one of the speedy zodiac rafts or rigid-hulled inflatables that zip in and out of caves and close to coastlines and generally speaking go much faster than the larger boats. These also take less people so you are likely to have more personalization on the trip and be able to ask more questions of the captain. If you possibly tend toward motion sickness, stick to one of the larger vessels, most of which are catamarans, that have a broad beam and are more stable on the water. These larger vessels will also have specialty items like bathrooms and running water and changing rooms.
One question we often get regarding our snorkeling tours is “do I have to bring my own snorkel mask?”. The answer is a resounding no. All the boats will have them for you and many of the larger boats will even supply optical masks that will approximate your level of eye correction necessary to see well, but if you are really wanting precision in your mask, bring your own. There is a good likelihood that the mask you get from the boat will leak on your face. This is actually unavoidable. For a one person owner the mask will tend to align itself with the features of your face and provide a non-leak seal. The masks on board any of the boats are of course used over and over again by people of all facial contours and they will often distort to a small degree and may not provide the perfect fit when you put it on.