What I expected to happen was...
Lowering the Sea Level setting would not allow water source blocks to generate above the setting's value, including at the new Ocean Monuments. I expected that if I lowered the Sea Level to 1 then I would see an Ocean Monument with no water on the inside or outside. If I lowered the Sea Level to some value in the middle of the Ocean Monument, then the water would only come up as far as the Sea Level I set, and there would be no water inside or outside the Ocean Monument above that y-value.

What actually happened was...
Water still generated inside and outside the Ocean Monument as a massive block in an otherwise completely dry ocean. See attached screenshot.

How to reproduce:

1. Create a world and locate an Ocean Monument.
2. Write down your coordinates and exit that world.
3. Re-create the world (to get the same seed), but before creating, change the World Type to Customized and set the Sea Level to 1.
4. Create the new world and /tp to the coordinates you wrote down from Step 2.
5. You will see the Ocean Monument surrounded by water in an otherwise completely dry ocean.

Here were the settings I used:

Note: I took advantage of this other setting on the Basic Settings screen to help me find an Ocean Monument. The seed and location above will not produce a Ocean Monument if you do not also change the Biome setting to "Ocean".