I would also watch the amount of milk intake, and make sure they are drinking lots more water than milk. I think by now your kid should be not having anymore than like 16-20 oz of milk in the day.

I worked with a dentist that specifically worked with kids for poor airways. Tooth pillow is a tongue and lip correction advise not specific to mouth breathing. It is not an expander, but its more around proper tongue position and behavior.

It won't work for some kids and you will still need an expander, but if your kid is young enough it could help.

I had a friend experience a very similar thing in the pool, as well as splash pads with her daughter. Most important thing was not shaming the behavior and making a huge deal about it.

They had lots of talks about how that is not something we do in public. She always made it a point to relate to her and call out that I know it can feel funny/good, but that is something that is never done in front of people. Just like we don't show our privates we dont ever touch our privates in public either.

After a few conversations, simple reminders (no shaming) it started to fade and she didn't do it as often.