That echo is annoying and if it lingers too long water in the ear can actually become painful. Especially for kids. Here is a tip that works every time but requires a helper. Have the swimmer lay on their side and pour about a half a cap full of rubbing alcohol in the ear. Let it sit for 5 to 10 seconds and then place a dry rag on the ear and sit up. Now do the other ear the same way. It works every time. If you take a cup of water and a cup of rubbing alcohol (Isopropyl) and a cup of water and pour them into a larger container you will not get 2 cups of liquid. Some of the liquid is automatically absorbed. This same principle works in your ears on a much smaller basis. The water stuck in your ear shrinks or evaporates when mixed with the alcohol and comes right out. If you get water in your ears give this a try.