Because:

not all females can use hormonal contraceptives and, unfortunately,
too few men choose to have vasectomies even when they don't want to
impregnate ever (or again);

condoms are not one size fits all shapes and lengths and do come off;

when you're trying your head above water and your partner's head above
water it's easier to forget to hold on to the rim of the condom during
withdrawal;

water tends to thin natural lubrication making intercourse feel "eh"
(null) to both participants or even uncomfortable;

water can be forced into the vagina by sex in water situations
(including in showers)and could dilute the contraceptive gel or help
dislodge a diaphragm;

being filled with water doesn't necessarily feel all that great
especially when you are sensitive to chemicals or very hard water;

when you get out, the water forced inside does eventually drain from
the body and it can be at an unexpected time (like when your ear
retains water and surprises you hours later);

your knees can slip off a ledge seat and crack your chin on the edge
of the tub (and the same can happen when you're hanging onto the edge
of a swimming pool) the blood and pain can be distracting and who
wants to explain how you chipped your tooth?

And it is _wonderful_ to relax in a sensual place (with or without
nudity) and not have it automatically become a sexual situation.  

Like being at a nude beach.  

The companionship will be valued far longer than feeling that sensual
must always lead to sex.  
Right now.  
Or even that same day.

If you cannot seduce out of the tub, being in one is not going to
magically change things.

LK
(who is not going to disclose how many are actual experiences and how
many are conjecture based on how much worse could have happen based on
the experiences)