I just spent 10 days on Cape Cod (it was an unheard of 90+ everyday) swimming in the ocean and only had one change (regular change schedule)
I use Coloplast Extended Wear wafer, an Eakin Seal and top that off with a Sure Seal (I do use Skin Prep but only right around my stoma)...I'm also a runner, kayaker and anything else that catches my attention...and have no problems.
Sure Seal is made by Active Lifestyle and you can find it through most suppliers.
My guess is either your soap has something in it (like moisturizer) or you are going too far away from the stoma with the skin prep which, when it gets wet, can release the wafer
In my early ileo days I did have that issue and it was resolved by limiting where I put the prep!
Good Luck, springostomy, and let us know how it goes...welcome to the forum, too!