I have no experience with making any kind of content on PMC or any other website/platform, and I haven't been able to find a datapack that makes Guardians spawn outside of Ocean Monuments; I'm thinking that they could spawn about 25 or so blocks below sea level in the Deep Ocean biomes, trenches and caves, a couple more welcome features would be to make Glow Squids spawn around the same height and place and to make oceans deeper by 5 blocks.
If anyone's willing to make this, send the link to the datapack in the comments of this thread once it's done.