I have a 15*15 farm. Its 64 to 46. full of water. It took so much time to do it. Your help will be appreciated. Here is my picture.
The water may be too small. You may want to get a bucket, and scoop up a squid (if possible). Then empty the squid into your pool. You can find squids in an ocean biome.
Try to go afk next to your farm covered by dirt. Mobs will only spawn in the chunk you're in.