Drowned spawn naturally at light level of 7 or less in all ocean biomes (except for warm ocean), and river biomes. They spawn at higher rates in rivers than in other biomes.

Where can I find drowned mobs?

In Minecraft, you can find the drowned in Ocean and River biomes. If a zombie drowns in water, it will also become the drowned. If you are having trouble finding a drowned, you can summon a drowned using a cheat or you can use a spawn egg.

How do you get drowned to not spawn?

You can place the coral blocks down under water and then put some sea pickles on the coral and bone meal the sea pickles. They will multiply and you can easily harvest them. Once you have enough they can be placed on any block and 4 pickles produce enough light to prevent spawning.