When you're using the flint and steel underwater, the durability of it is going to be used up. It should not be allowed to place fire with flint and steel underwater at all to prevent that.

How to Reproduce
1. Find any body of water, preferably oceans or rivers.
2. Use your flint and steel underwater.

Observed Result
The flint and steel can be used underwater but you can't place the fire, causing the durability to be used up.

Expected Result
You should not be able to use flint and steel underwater at all.