Well, went down to Cochin on the weekend. The water around the city itself looks positively dangerous to yer health. I'm not sure about municipal sanitation services there, but by my eyeball the water looked pretty polluted. Commercial divers operate several business there, and they do go into the harbour to work on ships, pipes & such, but I couldn't bring myself to hop in, even on the ocean side of the city.
There is a dive shop, Scuba Cochin, catering to the tank crowd, no freediving people or places that I could find.
The terrain is all beach to the north of Cochin, one of the locals told me it's the longest stretch of uninterrupted beach in the world. The water seems to be clean with lots of people splashing around and staying healthy
typical murky beach diving with low creature counts. The folks from the dive shop take their students out to a local freshwater rock sinkhole or quarry to do their checkout dives, and it seems they go on beach dives too.
There are reefs on the west shores of India, but you have to go about 100 km to the south of Cochin, didn't make it down there this time, but I would like to go sometime later, as well as to the Gulf of Mannar.