Minecraft – Protecting walled villages with large water sections

I think I know the issue but I'm looking to verify. My village is a coastal village and I walled the whole thing in. A good portion of what I walled in is water. For places where the wall goes over water I didn't build the wall all the way to the ground beneath – I just made sure the wall goes at least one block under water. In the earlier versions I played this was always sufficient, as any monsters that spawned outside just bobbed along the surface and hit the wall. The wall fully surrounds the village, has an exterior overhang, and 100% of the village is fully lit (the only exception is the water sections, which I'm not sure how to properly "light").

However, I came back to my village after a few days of mining underground and found that only a few villagers were left. I found a number of zombie villagers in the water, and, most importantly, I found one drowned inside a house.

I figure one of two things happened:

1. Drowned spawned outside my village, swam under the walls, and came out of the water to attack.
2. Drowned spawned in the water in my village and came out to attack.

The former is easily fixed by extending my walls all the way to the ground underwater (already done). The latter is fixed by…. lighting the water? Filling in the water with ground?

I'm hoping that extending the walls down to ground level will fix my issue, because filling in all the water will be a pain. However, I'm not sure about the spawning rules for these new Drowned, and how they might be getting in. I'm also not sure if I might be missing something else obvious.

So, what extra things might a village with large water sections have to worry about?