I used a river buffet world. Here's a picture from inside a large pool of water I created in the problem area. It should be noted that I am on a server, but we ensured the mob cap was not reached and also that everybody in the server was in this area. The server has nothing that would prevent mob spawnings, and we are on hard mode. I built the tub of water high in the sky, to ensure that the drowned were the only mobs being spawned.
I used a river buffet world. Here's a picture from inside a large pool of water I created in the problem area. It should be noted that I am on a server, but we ensured the mob cap was not reached and also that everybody in the server was in this area. The server has nothing that would prevent mob spawnings, and we are on hard mode. I built the tub of water high in the sky, to ensure that the drowned were the only mobs being spawned.