Ear tubes, or myringotomy tubes, are placed to help prevent middle ear infections (otitis media). If they have been removed (actually, they usually just fall out by themselves), then the doctor must feel that her risk of otitis media is low. Being in the water year round doesn’t increase your risk of otitis media — but it does increase your risk of otitis externa, often called swimmer’s ear.

It’s not clear from your question whether your niece has had trouble with swimmer’s ear. But to prevent it, ear plugs can be very helpful. It can also help to dry the ears out using a hairdryer (with caution, obviously) after practice, or to use a few drops of alcohol in the ear, which gets rid of the water (and then the alcohol evaporates). Alcohol drops for preventing swimmer’s ear are widely available in most pharmacies.