Make a void world (GoG, for example) and build a water tank in an ocean biome. Observe hundreds of guardians spawning non-stop (about 400/minute).

Found the problem. Their spawning is capped by squids. It seems that you are adding them to be spawned as a water type of mob while their actual type is hostile. So when there are no squids, the cap is never reached and guardians keep spawning.