Helpful, if the husband and wife were swimming together in a private setting, why would that be a problem?
There is no problem. If they can do other things together, surely they can go swimming together.
As an aside, the modest bathing suits that are sold (100% tzanua) do not get tight when wet due to the material they are made of.
Aqua Modesta makes such bathing suits, IIRC.
The Wolf