RN
Water does four separate things to sex, and only one of them is the thing people worry about. Sorting them out is most of what a useful answer to this question involves.
Before any of it: this is general information, not medical advice, and it cannot account for your circumstances. Anything specific — a rash, a possible exposure, a contraception failure, a pregnancy, a heart or blood pressure condition — belongs with a clinician or a pharmacist, not with an article.
The myth worth killing first
Neither chlorine nor hot water is a contraceptive, and neither is a barrier to infection.
If ejaculation happens inside the body, the pregnancy risk is the ordinary pregnancy risk. The surrounding water is not involved. Chlorine at the concentrations used in a maintained pool is not sitting inside anyone, and heat at hot tub temperatures is not doing anything useful to sperm that has already been deposited.
Sexually transmitted infections work the same way: they pass through direct contact between people, so the contact is the exposure and the water around it changes nothing about that. The related worry — catching something from the pool because other people have been in it — is a different question and a much smaller one, since properly disinfected water deals with most of what would otherwise be a concern.
So the water neither protects you nor, by itself, infects you. What it does is interfere.
Mechanical: water is not lubricant
This is the least dramatic problem and the one most likely to actually spoil the evening.
Immersion washes away natural lubrication and does not replace it. Water is thin, it does not stay where it is needed, and the result is friction — which at best is uncomfortable and at worst causes small abrasions. Broken skin matters, because it is a more receptive surface for anything being transmitted, so a comfort problem quietly becomes a risk problem.
Water-based lubricant is pointless here for the obvious reason. Silicone-based lubricant persists in water, which makes it the sensible choice — though it should not be used with silicone toys, which it degrades. Oil-based products should be kept away from latex entirely; that rule does not change in water.
Condoms: the honest position is that nobody knows
This is where confident advice tends to outrun the evidence, in both directions.
What is reasonably established: condom manufacturers test and rate their products for ordinary use, not for immersion in chlorinated or bromine-treated water at elevated temperature. Latex is sensitive to heat and to some chemicals. Loss of external lubrication and the movement of water both make slippage more likely.
What follows from that is not "condoms fail in water" — that would be asserting a result nobody has measured. It is that a condom in a hot tub is being used outside the conditions it is validated for, and its reliability there is unknown rather than reassuring. If pregnancy or infection prevention is the point, that uncertainty is the reason to do the part that depends on the condom somewhere else.
Two practical notes if you use one anyway. Put it on out of the water, where it can be applied to dry skin and correctly. And check it afterwards, because slippage in water is easy not to notice.
If a condom fails, emergency contraception is time-sensitive — the sooner the better, and the options differ in how long they remain usable. That is a same-day conversation with a pharmacist or clinician, not something to research at leisure.
Biological: what warm water grows
Hot tubs specifically have a well-documented problem, and it is worth knowing because it is common and easy to misread.
Pseudomonas aeruginosa is an ordinary environmental organism that causes what public health guidance calls hot tub rash — folliculitis appearing typically a few days after exposure, characteristically worse where a swimsuit held water against the skin. It usually settles by itself, but a rash after a hot tub is a thing to have looked at rather than ignored, particularly if it worsens or you are unwell with it. The same organism is behind the ear infection swimmers get.
The reason hot tubs produce this and pools mostly do not is chemistry under load. Disinfectant is consumed faster at hot tub temperatures, the water volume is small, and the number of people in it relative to that volume is high. A domestic tub that gets topped up rather than tested is very often not holding an effective disinfectant level at all. Adequate free chlorine or bromine deals with this organism; the problem is that "adequate" is not the default state of a poorly maintained tub.
There is also a widely reported pattern of irritation, thrush and urinary symptoms after sex in treated water — a plausible consequence of chemicals and water being moved internally, and of the disruption to normal lubrication. It is reported more consistently than it is measured, which is how it should be presented: common enough to expect, not something to self-diagnose.
Thermal: the hot tub is the actual risk
Of everything here, sustained heat is the part that can put someone in genuine trouble, and it is the part that gets treated as ambience.
Hot tubs are generally set at or near the upper limit conventionally recommended for domestic use, and prolonged immersion at that temperature raises core body temperature. Exertion adds to it. Alcohol adds to it, blunts the warning signs, and impairs the judgement that would otherwise get you out.
Dizziness, nausea, a racing heart, headache, or feeling faint are the signals to get out, cool down and drink water — not to push through. Overheating in a hot tub also carries a real drowning risk if someone loses consciousness in the water, and slippery surfaces around it produce the ordinary injuries you would expect.
Two situations to take to a doctor rather than an article: pregnancy, where raised core temperature is a recognised concern and the standard advice is more cautious than for anyone else; and any cardiovascular or blood pressure condition, or medication affecting either, where hot water immersion is not a neutral activity.
And the one that is not medical
A private tub is a private space. A hotel pool, a gym spa, a shared complex, a lake, a beach — none of them are, and other people, including children, are entitled to be there without encountering this. That is a genuine boundary rather than a squeamish one, and in most places it also carries legal consequences that are disproportionate to the moment.
The short version
Sex in water is not dangerous in itself. Hot tubs are the version to be careful about, and the reasons are heat, disinfection and alcohol rather than anything to do with sex.
If it appeals, the low-friction approach is the obvious one: use the water for the part that benefits from it, bring a silicone lubricant, and do the part that depends on contraception or barrier protection out of the water where the equipment is being used as intended. Rinse and dry properly afterwards.
For the contraception question in general, our guide to birth control methods covers the ground properly. For anything you are actually worried about after the fact, a clinician can settle it in one appointment and this article cannot.