In residential applications we don’t see expansion joints in the bottom of a concrete pool. We do see them between the coping and patio of concrete/gunite pools and use Sikaflex. If you have two surfaces that will hold it in between I’d expect the same product could be used. The trick is to have a trough/valley it can be put into since it is self leveling it will find the lowest spot and settle/level out.

In larger commercial pools it would seem more likely to have expansion joints in the bottom of the pool, and in some cases cracks could form. There’s a company that specializes in products for in pool application, Sani-Tred. You can view the Sani-Tred website here.