In addition, it is unwise in tropical countries to swim in lakes or rivers because of the risk of schistosomiasis. This is a disease caused by parasites which enter through the skin. It can result in bowel or bladder disease, chronic liver or kidney failure and even spinal paralysis. Even brief exposure to infected waters such as by rafting or fording barefoot through a stream may be sufficient to result in infection. There are other infections as well, most notably some deadly amebic infections, which can be caught by swimming in infected waters, even in temperate climates.
Snorkeling is another favorite water pastime.
This is fine with appropriate supervision and instruction but again, beware of unfamiliar waters. Waves and currents may be hazardous and certain marine plants and animals can inflict painful and even dangerous injuries. Rubbing, scratching, toweling and washing with plain water may make the situation worse. Injuries from manta rays, biting or stinging fish and shark bites can cause infection or serious tissue damage and usually require prompt medical attention. Scuba diving, of course, carries a number of particular risks and should be undertaken only with proper instruction and supervision.