In 21w13a ocean monuments generate empty, there are no rooms and it is just filled with water. Guardians spawn, but elder guardians don't. 

To recreate:

1. Load up a new world in 21w13a (creative mode)
2. Use the command /locate monument to locate a monument
3. Teleport to the location
4. Break into the monument

Seed and coordinates I used:

2364154432644256842, -1867 44 817

-8992078913880377694, 1732 45 -2209