Is pure hyaluronic acid mixed with water okay to put on my scars?

Any wound management postoperatively is the responsibility of your surgeon. I personally cannot see any benefit in putting hyaluronic acid on a healing scar. Conversely I don't see any harm.

This question is best answered by your surgeon as he/she is most familiar with the details of your surgery and how you've been progressing through recovery.  Scar treatments vary and your surgeon will need to verify that your incisions are healed to the point that beginning scar treatment is okay.