Although I haven't had an ostomy in 54ys I have a k pouch that occasionally needs to be hooked up (and the stoma covered for certain activities )
I am handy with a needle and thread as well as having a wide imagination when it comes to creating clothing that works for these occasions.
If you have a bathing suit that is rather thin, a nice fabric but a bit 'used' or no longer adequate for your body, don't throw it out...cut out the crotch from the bottom, fold it under and hem it to make a tight, swimmable abdomen cover (kind of like a spanx tummy tuck thingy...don' t know the name exactly) that you can wear under another bathing suit to hold your bag tight, in the hot tub or any other wet zones.
I did the same with some stretchy, sexy undies and one piece body suits...In the days of snappy crotch 1 piece lingerie, it made a huge difference to my personal comfort zone...I had sexy, silky lingerie that covered everything nicely (I would just unsnap the crotch until they invented large, wide garter belts that covered the whole abdomen...then I would just remove the thingies that held the stocking up and keep what looked like a sexy, extra wide belt)...I am very, very self-conscious about my abdomen and stoma so the lingerie gave me the freedom to feel 'normal'...and not reveal too much before I was ready...I even managed to maintain very long term relationships with guys who never figured out that I had a stoma unless I was ready to tell. One of them I ended up marrying. (finally had to tell him after a few years...he had figured it out by then and didn't care)
Sharon