We just got back from a cruise out of San Juan (different cruise line), and stayed 1 night before the cruise at the Marriott. Very nice hotel, well located, clean, large room. There are 2 restaurants in the hotel, one a fancy Italian and the other a buffet/menu order combo, with some tables outside on the terrace. Not cheap, and the buffets we thought were quite good & well worth the money. There are also some decent restaurants walkable from the hotel, I understand, but with rainy weather and late arrival, we didn't bother. The hotel is a lively place, at least on weekends; the disco, which is open to the lobby, is VERY loud at night; casino is good-sized, located just off the lobby. Nice pool area, and a nice well-groomed beach (assuming the surf lets you use it.)
As far as tours and things to see, it will depend on how many days pre- and post-cruise you are staying there, and on your air schedule, as to what to see. We had a late flight back so opted for a rainforest tour after the cruise, which was enjoyable - beautiful scenery despite the rain (in fact, that added to the atmosphere, we thought). The bioluminescent bay is a long way - I don't advise doing it on your own unless you have at least 2 nights before/after the cruise. And if the seas are at all rough, tours there will most likely be cancelled.
Just be sure you don't have to spend more than a couple hours at the airport, if at all possible. There isn't enough seating anywhere, and no decent food past the security checkpoint!