Hi Nay,
The diving ENTs that I know unanimously recommend against hydrogen peroxide; about 1/4 recommend against alcohol & most that do recommend alcohol say keep it below 50%
Used repeatedly, alcohol & especially hydrogen peroxide will dissolve the cerumen (ear wax which is both chemically & physically protective) in the external auditory canal and can be excessively drying, leaving the canal sore & prone to infection. Remember, this is on top of the drying action of repeated, extended soakings of the ear in water while diving.
Placing a few drops of mineral oil or lanolin into a well dried ear canal before each dive may be a better idea.
If one wants to go with a post-dive mix, a mixture of 1/3 white vinegar, 1/3 alcohol, 1/3 distilled water & a thimble full of mineral oil is sensible. Shake well before using.
As for treatment, it somewhat depends upon whether the infection is bacterial or fungal in nature, but combination preparations for the ear containing neomycin, polymyxin B & hydrocortisone are often a good place to start when recommended by a doctor.
The diver prone to infection of the auditory canal will want insure that there is no excess build up of ear wax. He will also want to keep the ear canals dry through mechanical means. These might include gently towel drying as thoroughly as possible between dives & carefully airing the canals with a hair dryer at a LOW heat setting following the diving day. BTW, the overly intrusive or vigorous use of cotton swabs generally is not recommended.
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual, and should not be construed as such. The diver will want to consult with an ENT prior to any endeavors at the prevention or treatment of otitis externa.
Best regards.
DocVikingo