25 Water drops
Start by moving the fireflies then you can get to the water. Once they are gone, make as much water as you can which will hopefully jump up and break the iced crops freeing the mushroom.
Once you have liberated him, make rows next to him until you have your 10 needed.
While making as many possible rows as you can next to your water buckets, keep an eye out for suns. Always grab the row of suns first with any points on for easiness. They are harder to get.