Season period
Puerto Rico located in the caribbean, benefits from storms, cold fronts, and low pressure areas to provide good waves almost year round. In Puerto Rico surfing season starts in October and ends in mid-April. The month of May still brings some ocassional swell, but not as consistent and good as the normal season period. On June 1st, hurricane season starts, till November, so basically there is a chance of encountering some waves all year round.
Breaks
The best and most consistent surf areas in Puerto Rico are the north, northwest, and west coast of the island. The are also some good surf spots in the south which are the ones that work around summer time (June-September) like la escuelita, inches and others. Some of the most famous spots around the island are: Gas Chambers, Chatarra, Tres Palmas, Aviones, Maria, and Hollows, but the island actually has well over 60 breaks.
Sea urchins and sea lice
A good amount of the breaks are reef breaks so expect some sea urchins on places like Marias, Wilderness, Middles,Dogmans, Surfers Beach, Aviones and some other spots. Also in the past couple of years sea lice (it's actually a sort of small jellyfish) have been found in some areas, although not as serious as in Florida and some other caribbean islands. Sea lice are small animals that look like a pepper speck and come in schools. This small animals get trapped in your clothing or board when you pass over them. The problem is when your body has friction with your clothing or board. To protect yourself wash your bathing suits at least with water before using it again, and try washing off or changing clothing before going into a car (as contact with seat might cause sting). To treat eruption simply use of creams like hydrocortisone or Benadryl which will relieve itching.
Dangers
Although most breaks a reef type, Puerto Rico is a relatively safe place for surf. The usual first aid stuff will do, specially bandaids for small cuts. But remember Reef wise the danger increases exponentionally with size, specially on low tide. Another big issue, one that's most dangerous for inexperienced surfers, is riptides, which also gets more dangerous with size. These two are the most serious dangers in the island. Places to watch for rips: Jobos, Wilderness, Marias, and Surfers Beach. The Atlantic ocean has plenty of sharks, but there is no reports of shark attacks around Puerto Rico.
Good for snorkeling too
Most of the island benefits of beatiful clear water, so if it's flat, theirs always snorkeling and scubadiving. One of the many good places for snorkeling is Cabo Rojo at the southwest coast of the island.