You may try breaking up the optics of the water surface to it doesn't look like a nice open poop dropping spot. Maybe some noodles or javex jugs on the top.

If you do some searches on Grackles, there is some info out there. We had them doing this for a couple of years, then a neighbour took out a couple of nests that were on his property and they relocated elsewhere since then.

http://www.massaudubon.org/printwildlife.php?id=15

"NEST SANITATION

Birds have different methods of keeping their nests clean. The young of some species defecate over the edge of the nest, the adults of other species eat the fecal sacs, or carry the sacs away from the nest and deposit them in vegetation or water. Grackles dispose of the fecal sacs of their young in water and to a grackle, someone's swimming pool may just be the closest body of water."

SITUATIONS AND SOLUTIONS

As stated previously, nest sanitation can take on different forms depending on the bird species. Common Grackles use their bills to pick up the fecal sacs eliminated by their young at the nest and dispose of the sacs at a nearby water source. That water source is often a backyard swimming pool.

"SOLUTIONS
When not in use, cover the pool with a lightweight cover that can be easily removed.

Be patient, this behavior will last only as long as the young are in the nest, about 2½ weeks.

Do not waste money on fake owls or snakes, or spray repellents - there is nothing that will prevent the birds from flying in the "air space" over the pool.

It is impossible to keep birds out of trees, but if the nest sites are known, thinning the trees in the fall, after the birds have left, may make them less attractive as nest sites the following spring."