All bags are designed to be waterproof. It's just that certain bags stick better to certain people while swimming than others. It's really a matter of experimentation. What works for one person may not necessarily work for another unfortunately.
Some like to 'picture frame' their wafer with water proof tape, others don't. Some use a blow dryer on it's lowest setting to dry the wafer off afterwards, others don't.
Try soaking in a bath for 1/2 hour to see if your bag is up to the job. If it does come loose, I would advise you to ring/email ostomy manufacturers and get as many free samples as you can to try.
I have had an ileostomy for 30 years now due to UC.
I'm not a complete idiot - some parts of me are missing!