Water conditions don't really change much on that end of the island. It will be hot, sunny, dry, and windy. Water is always clear and calm. Are you going to be in La Parguera, Guanica, Cabo Rojo?
Here in Aguada and Rincon, the visibility varies drastically in the summer. When it rains, all of the rivers dump out mud and pollution that the Puerto Ricans inland dump into the water. When we go a couple of days without much rain, the water is clear.
Down in the southern end of the island, there is very little rain, and they don't have the rivers dumping into the ocean, so it's not as much of an issue. I would expect your trip to be par for the course.
A tip: wear plenty of sunscreen. I have been sunburned in under 12 minutes just hosing off my terrace.