Check out this Blue Melody/Black Pearl. I've gone out with them at least three times over the years with my family and have had a wonderful experience each time. It's much smaller than Jolly Pirate's. We've seen young children, but I'm not sure if they were 18 mths. The youngest I remember was probably around 3. He had a great time.
I'm glad you found the Black Pearl/Blue Melody site useful, cristina. If nothing else, you can get a very good idea of what to expect on their tour from the pictures and videos. The owners are lovely people, and if you have any questions, I'm sure you'd get a prompt reply if you send them an email. Personally, I much prefer a smaller quieter boat to the large party boats, but I know I'd particularly prefer it if I were traveling with a toddler.
The fresh grilled tomato/cheese sandwiches they serve on the cruise are simply delicious, and those followed with the watermelon are more than enough for a meal. My sons aren't big tomato eaters, but they love those sandwiches. So, in the end, the cost of the boat/snorkel trip is greatly offset by not having to go out for dinner.