It is extremely dangerous to be walking in dark when there is no foot traffic, and if you find yourself in this situation, it is recommended you find a taxi to go to wherever you need to go.
Like any other tourist destination, watch out for pickpockets. Buses and bus stops are frequent locations for robbery, especially those destined for San Jose. A money belt with your passport, cash, cards and tickets is a good way. Purse snatching, armed robberies and car-jacking have been on the rise lately. Stay alert and protect your valuables at all times, especially in the San Jose area. Do not leave valuables in your vehicle, or if you must, make sure they are not visible.
Costa Rica has some of the best beaches in the world. However, the public beaches do not have life guards, so traveler should learn how to swim out of a rip tide and not swim alone. Crocodiles are quite common in certain parts of Costa Rica. The biggest spot for them is the Tarcoles river bridge in the central pacific as posted in the Jaco wiki. It is recommended to stop the vehicle nearby and walk across it. Great care should be taken when swimming or snorkeling, especially near areas where fishing is common or near river mouths.
Emergency number: 911