How can I prevent tankini bottom from falling off when diving into pool?

I go to the city pool a lot and I wear a tankini. MY mom always said the bottom was too small, but whenever I dive, the bottom falls to my knees. How can I prevent this from happening
P.S. I cannot get a new suit or sew it so it fits better.
I would really like to continue diving :-/

I would just stop diving unless you want to wear a one-piece. It's going to keep happening.

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