I have used the same mix. 50% white vinegar and 50% alcohol (drugstore type). It's not prophylactic in the sense of using it before a dive, but what you do is use it after the dive. I fill up one ear, lie down, and wait five full minutes - then do the other ear. I have the solution in a dropper bottle.
I first read about this in a medical book for cruisers, i.e. sailors who may spend quite a bit of time in the tropics, and may need to self-medicate.
It's one of those things that is hard to put a number on how well it works, since you don't know what might have happened if you didn't use it, but I swam pretty much every day in the tropics for the better part of a year and by using this routine religiously did not get any ear infections. The book said it was important to do the full five minutes in each ear.
I believe the vinegar kills the "bugs" and the alcohol dries the canal (which also helps).
Blue Sparkle