My husband usually ends up with an ear infection whilst on holiday. It is usually caused by the swimming pool and him not drying the insides of his ears properly when he comes out and spending too much time underwater messing about with the children! Do ever be tempted to dry ears out by put Q-tips into the ears as this just forces it further moisture further down the ear canal. Just use either a rolled up piece of tissue or the corner of a towel.
We were told never to use ear plugs as these too can trap the small amount of water than manages to get into the ear and hence cause ear problems.
The only method we have come up with that seems to work (and was recommended to us by someone who was deaf because of a serious ear infection) is to cover the outside of the ear with vaseline and put a small amount of cotton wool inside the ear and then smear some vaseline over that. We tried this last year in Tenerife and it certainly worked for us.
Good luck.