Apparently not everyone has this problem, but after I swim I get water stuck in my ears and it can be miserable. It has also caused ear infections that have been horribly painful.
A year or two ago I saw a nurse practitioner about the problem. She was mostly entirely unhelpful but told me that some people just have more of a problem based on their inner ear shape. She gave me some massage type practice to get the water to drain. It didn’t work very well.
What I have found DOES work is to put a little bit of alcohol in each ear after swimming. It works every time! Just enough so I feel a seal. I let it sit for just a couple/few seconds, and then drain, wiping up with a tissue. Then I do the other ear. I have been doing this every swim for at least the last year with no negative consequences; just happiness not to have the nagging water stuck in my ear!
A different but related problem – many people find that they can get dizzy coming out of the water after an open water swim; and that this can be alleviated by wearing ear plugs. This would also help keep the water out during a race when you will not have time for the alcohol trick. I have done this in races past and think I will make sure to pack some ear plugs for my coming race!