I can't ride horse

Do you sink and then automatically dismount? If so, this is expected behavior.

If the horse is in water that is one block high or less, that is possibly a bug.

Thank you for your report!
However, this issue has been closed as Working as Intended.

As mentioned, riding in deeper water will cause you to dismount. (More information can be read on the Minecraft Wiki.)

If you need help or have a suggestion you might like to follow the community support link below.

Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Feedback – 📖 Game Wiki