I've had an ileostomy due to UC since I was 28 ( 38 years ago ).
I have swum dived, hiked, sledged, played cricket, rugby, cycled, used the gym etc.
I have never, through choice, exposed my bag to anyone ( apart from wife & medical staff ).
It's easy to get dressed & undressed with a towel wrapped around you & to wear your costume or a pair of shorts in the shower. If people remark that you are " shy", either tell them the situation or tell them to P*** off.
I have been lucky not to have any problems since the op & although it goes without saying that I would rather not have a bag, it has given me a normal life after suffering eight years of debilitating illness.
However I realise how really lucky that I am after reading some of the anguish & suffering of people on this site, it just makes me want to weep for them & I feel that I have nothing to say which will help.