When you take a walk through the Minecraft plain and you look at the ponds there is probably a drowned in there. So can the drown just spawn in the ocean because at night on a normal plain far from the ocean the drowned just walk around. Maybe just make the drowned only spawn in the ocean.
This should already be the case. Drowned only spawn in rivers and in ocean biomes. Those that you see in other biomes either walked there from wherever they spawned OR a regular zombie walked into the pond and died a watery death (they converted to Drowned).