Coating is on too thin.

This sample is about 5 mil it needs to be a minimum of 20 mil for underwater applications.
This has pin holes water gets behind the coating and works it loose

20 mils is 40sf per gallon its not the number of coats.

Dura-Rubber is 60% solids you will apply 34 mil wet to get 20 mil dry.

If you have 40sf your apply 1 gallon until it is gone.

This is normally one thin first coat and two additional coats.

This can vary depending on the texture of the surface you are applying in on.

Solution: Apply per application directions for pools.  POOL INSTRUCTIONS