It may help to mix the fertiliser into a ball of clay and put that in a hessian bag of aquatic compost around the roots. The roots will enter the clay and access the nutrients there, rather than them leaching into the water.
The RHS says the following; Waterlilies are heavy feeders and benefit from a proprietary aquatic slow release fertilizer added into the compost or soil during the growing season, following the manufacturer’s instructions.