So the thing with the Southern out of San Juan cruises is that they are port intensive. There is often maybe 1 sea day, so liking or not liking the ship can be limited to where they sleep and the food, all to say the ship means less on these cruises than on a longer cruise with more sea days. Personally, I enjoy the food on Carnival, but of course that is subjective. Fascination does have a serenity deck, and Carnival always has "family rated" comedy shows that seem quite popular. They do have other shows as well but I have never heard them called the best at sea or anything
Not sure what time of year they are looking at? RCL Freedom of the Seas cruises from San Juan as well, and Celebrity does as well, not sure of the ship, And NCL who I believe is moving the Epic there (which is a unique ship with an interesting cabin layout), if they are looking at other options. But Carnival would be the cheapest of those.