1)"the walls are water!". Any walls you put up are simply very narrow glass tanks filled with water. Guardians could spawn throughout.
2)sewer system. Be like a ninja turtle and live in a network of stinky tunnels. The tunnels you do stuff in would be dry, you could intersperse smaller inlet tunnels that spew guardians out after falling out of the main spawning area.
3)"Dr. Fishin'stein's Monsters". Mad Scientist type of lair with specimens kept in stasis chambers, maybe a large shark-tank for you to monologue over while the hero escapes and thwarts your plans.
4)MC Dept of Conservation. Probably-modern-esque build style, the base is basically one gigantic viewing area for the larger Nature Preserve. Smaller areas where the scientists are working can be hidden away where the tourists won't notice.
5)MCS Nautilus Charm. A gigantic submarine a la SeaQuest.
6)Cannery Deep. Basically a factory hell-bent on processing guardians.
You could build a series of underwater domes and other areas with connecting passages around the farm, that means a lot more water clearing tho. Assuming you want to maximise the farm potential, the farm with be 50+ blocks square (cant rem exact size of spoawning zone for guardians abound the monument) so ideally your base will be around the outside of this perimeter.
If you have land close by, why not build there and just visit the farm by underground minecart link when you want to use it? I guess if the farm is permanently on, you will want to be in the surrounds at all times, but if its just one with an on/off mechanism you can visit and AFK when required, so you could build away from it.
Below are some pics of my world where I have an underwater style base at one location, and a main base on land near my Guardian Farm at another
How about an Atlantis-inspired sunken city around it? Or a tower that starts on the seabed and rises up out of the water? In fact, several towers connected together, like but not like a castle, could be cool, too.
This would be a lot of work, but soaking up all the water in a radius around your farm, building walls to keep the ocean out, and then building on the seafloor as if it were dry land can also be a cool effect.