Again, without a perforation water can't pass through the tympanic membrane to cause otitis media or collect. Water can collect in the external auditory canal, but that wouldn't cause otitis media. As DDM suggests, it sounds like an anatomical anomaly, or a chronic infective process. Why it comes to the fore after diving, I don't know. EDIT: Could be an ascending infection from water contaminants entering the eustachian tube via the nasopharynx. Just a guess, I know it happens, but I don't know how common it is. I'm not a diving doctor or ENT surgeon.