The water couldn't get into your stoma - the peristaltic motion of the stoma makes it a one way thing. Things can come out but not go in - that's why suppositories in the stoma aren't a good idea, they'll just be pushed out again.
However, having an exposed stoma inside your swimming costume might not be the best thing to do as it will get messy esp with an ileostomy. Just wear your flange and bag as normal and go swimming instead :)
I have had an ileostomy for 33 years now due to UC.
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I'm not a complete idiot - some parts of me are missing!