There are mountains in the middle of PR, including the Tropical Rain Forrest called El Yunque, but if you are close to the coast it is very much like Florida. In a ton of areas you can go quite far inland before you are into the higher elevations. All of the tourist area where the hotels are in the northern part of the island, like the Condado area is right at sea level, including a bunch of homes. I attached a photo of the Condado area which is in San Juan, and right next to old San Juan, you can clearly see the thousands and thousands of people that live here at sea level.