I've just been diagnosed with this, in the outer ear, and have antibiotics by way of treatment. It's very sore and swollen, hence I have tablets rather than eardrops.
I'd wait a while but when I was a kid I often had otitis and wasn't allowed to opt out of swimming (which I hated) so had to put cotton wool in my ears under of course the obligatory swimhat. This kept the inside of my ear dry.
Oh God, yes, the swimhats. For the boys too, if they had long hair. :-D
The official advice from the Guardian BMJ site is around ten days, so I'll give it a couple of weeks off. If it's all gone down by then, I'll use earplugs for a few sessions.
Believe me, I have no wish to suffer again what I've been through this weekend.