You could use a clear polyurethane such as Varthane Spar urethane. It wont change the appearance of the wood much and provide good protection.

Wax isn't a bad idea. Keep it up though.

Wax spots, though.

You should never place wet items on shelves. You will damage them. No wax will save them. And it is a denied warranty claim because that is considered abuse.

You only place dry items on shelves. Emphasis place. No dragging or hitting the edges. That too is abuse.

If you wanted to use shelves as a drying rack, they needed to be ventilated stainless steel.

You could cut a piece of clear plastic to place on the shelf.