How to protect sensors used on atheletes?
The only way this is ever done is with some kind of product casing. You can buy pre-fab cases for electronic projects, but to get something really slick and appropriate for your purpose, you're gonna have to make something yourself. You might look into 3D printing.

Something like this might work: https://www.sparkfun.com/products/8601

For protection from salt-water, this product is worth a look: http://www.corrosionx.com/

There's some different products there, but I've seen a demo of that product, where we ran an entire RC airplane system, servos, motors, and receiver, UNDER WATER.