One of the Strongholds in my world (generated in 1.6.4, currently playing in 1.8) is under the seabed. On reaching the seas near the stronghold, I discovered guardians. There are no Ocean Monuments nearby (nearest one I know of is about 2000 tiles north of the stronghold). I can confirm these are spawning here and not moving here from somewhere else; I switched to peaceful, made sure the guardians did despawn, then switched back to hard difficulty, and observed guardians spawning in the water above the stronghold.
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Hm, I went to another Stronghold, also under the sea, to check if this happened there too, but no guardians spawning there. Therefore, it does seem like a dupe of MC-65214 as Sonic has pointed out... I'll try generating the world with the same seed to check if a temple is genned on the coordinates.
Dupe of MC-65214