Water Dragon Armor is a set of four items which can be obtained by attackingLakes level 7 and higher with a Great or Elemental Dragon as part of the attack. Alternatively, they can be purchased from the shop. Once you have obtained all four pieces, have Outpost 1's Dragon's Keep at level 8+ and have Aerial Combat at level 1+, you will be able to include the Water Dragon in your attacks.
As the items have a small chance of being found on each attack, many attacks will likely be necessary to find the set. Researching Aerial Combat, upgrading to higher level Dragon's Keeps and unlocking other Elemental Dragons will reduce the minimum time between attacks.
To reduce heal times, it is suggested that you attack the lake 2x without the GD/ED to kill all the anthropus and then use your GD/ED on the 3rd wave to get the armor. If you alternate waves afterwards using normal troops and then using the GD/ED + troops, you will do less damage to your GD.