If you fill and empty with raw seawater on the regular then you will have plenty of opportunity for growth on the inside and a constant supply of new water with fresh nutrients meaning everything is going to want to grow into the water inlet and clog it.

Maybe they will sink a giant bladder of sterile water together with the hollow sphere, and then figure out a way to make the bladder not fail for 20 years?

I envision issues with turbulent flow over surfaces that work best with laminar flow. It sounds like a turbine or pump system is used for these spheres.