The Water Main's healing jutsu heals the person with the lowest HP, regardless of whether the ninja is at full health or not.
If everybody is at full health, she'll heal the person with the lowest health of the 4.
If, say, a Naruto has 12k health and your main has 10k health, the water main will heal the main. If your Naruto is hit and is sitting at 11k health, the water main will still heal herself, as she has lower health.