I don't know if this is a bug or if it's just not possible to do, but It should be possible to make farmland using hoes, and grass paths using shovels under water to quickly make grass turn into dirt.
MC-134998
Can't make farmland/grass path blocks under water.
Resolved
Invalid
0
This issue has been closed as invalid.
You have posted a feature request or a suggestion. This site is for bug reports only.
For suggestions, please visit Minecraft Suggestions on Reddit.
Quick Links:
📓 Issue Guidelines -- 💬 Community Support -- 📧 Customer Support -- 📖 Game Wiki