Ocean monuments are quite productive usually even if you don't clear out all the caves, don't clear out all the water, and you have a relatively small covered "fish tank" that the guardians can spawn in.
Caves - it's the bottom of the ocean, so there's not that much room for caves in the first place.
Clearing out all the water - Guardian spawn rates are greatly inhibited by being exposed to the sky, so extra water doesn't matter so much.
Small "fish tank" - There are so many spawn spaces in a even a smaller fish tank, that the spawns in the fish tank tend to dominate all other spawns when you're nearby.
So if for example you clear out the room with the gold blocks and extend it upwards to the surface and cover it, you can make a guardian spawner that gives you what you need for most single player needs.
Guardians don't spawn under the floor (between the legs) so just leave it as water, then.