this probably shouldnt go here but oh well. might seem an odd question. im doing swimming as part of my college course which i start next month. I havent been for about 3 and a half years so i need some practice. anyway i was wondering how much time id have to spend in the pool before it would affect bone density? would it have to be several hours at a time? am i overeacting here?

you're overreacting. Unless you spend A LOT of time in the pool, it won't matter. You'll probably be on land way more than you'll be in the pool, so your bone density shouldn't be affected.

Why do you feel your bone density would be effected?

probably because when you're in water you're sorta "weightless" and your bone needs load on it in order to grow or just stay the same. If you were to spend extreme amounts of time in the pool, you'd probably lose some bone mass.

Simple fix: do some squats.

You forgot the obligatory "and milk"

I'll have to consider giving you an infraction for that

Along with everything else these guys have offered....

If your instructor is worth anything, he/she will be having you do dry land exercises too.

These are going to include:
body weight lunges
body weight squats
stretching exercises
leg kicks to work your core (hands under your butt, feet at: 10 sec - 6" off the ground, 10 sec - @ 45 degrees, 10 sec - straight up in the air... repeat backward for full minute and do this 3 times in a row.)
running and/or cycling

Not to mention any weight/resistance training you will need for shoulders, lats, delts, biceps & triceps.

If you have not guessed... having a kid on a swim team taught mom a trick or two