Semen is not typically impacted by water in the short term, which is to say that sperm will generally survive, at least at first. A lot depends on temperature as well as the amount of water present. In most men semen is itself made up primarily of water. Adding more will cause the mixture to become diluted, though depending on the specifics of the encounter the semen might actually gum up or become clumped at first if the water washes away some of the proteins responsible for keeping the mixture fluid. Extended water exposure may also lead to sperm breakdown over time. Sperm already have short lifespans, though. Sometimes warm water will actually prolong their life, but cold pools and puddles often have the opposite effect.
The semen of humans and most animals is made up primarily of water — up to 90% by some estimates. From this perspective, nothing unusual happens to semen in water, since in most cases it is already basically there. Human semen is typically a combination of water, sperm, and proteins, though, and one of the main impacts of added water is an upset of this balance.
Sperm generally thrive best in water that is approximately body temperature, or 98.6°F (37°C). They can survive in water that is slightly warmer or slightly colder, but not always. In these cases, though, it’s the temperature more than the water that’s killing them off.
Some men also find that their semen tends to coagulate or become almost gelatinous in the water, particularly in the shower. This is usually due to the way the water is hitting the ejaculate and the response of certain proteins. Two of the most important proteins present control coagulating and de-coagulating, often as an evolutionary protection mechanism. It isn’t true for everyone, but in many cases the de-coagulating protein will wash away first, leaving a gum-like mass behind. Over time this, too, will dissolve and wash away, but it can be unsettling to see at first.
Consequences of Mixing
In nearly all cases water will slowly mix and combine with any fluids added to it, and semen is not usually any sort of exception. Semen tends to be thicker than water, so this may take several minutes or even longer to occur. A small amount of semen may be seen floating on the top of water for quite some time after ejaculation, or it may sink. The ability of semen to float may be related to the diet and lifestyle of the man since eating certain foods or engaging in certain activities may impact semen's consistency.
It should be obvious that watery semen will dilute with water more quickly than will thicker semen. Thicker semen is more likely to appear white and float above the surface, while clear, thinner, semen is most likely to sink. The temperature of the water may also make a difference here, as well.
Pregnancy and Disease Concerns
Some women may become worried about pregnancy should they encounter semen in water, particularly during sexual activities performed in a bathtub or hot tub. This is not typically a concern, but if continued sexual activities are engaged in, the risk may be higher. If pregnancy is a concern, then protection should be used even when engaging in foreplay in the tub, pool, or hot tub. Although sperm won't live very long in cold water, they may survive for up to a few hours in hot or warm water since they thrive in wet and warm environments. It isn’t usually easy to become impregnanted simply by being in close proximity to semen, but a woman who is already naked and aroused may run a greater risk.
Some communicable disease-related concerns over semen in water are also common. Human immunodeficiency virus (HIV) is a major health concern, and if infected semen comes in contact with an open wound, the disease may be spread. This is generally not a problem in places like swimming pools and hot tubs if the proper chemicals are used in the water for cleaning and disinfecting. Other areas may not be as safe, although the chances of transmission from waterborne semen are still relatively small.