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.
Linked issues
Comments 1

Thank you for your report!
We're tracking this issue in MCPE-31324, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki
-- I am a bot. This action was performed automatically! The ticket was resolved by one of our moderators, and I left this message to give more information to you.