Water is an essential element to most life. Planets with large resources of water (especially fresh water) are valuable and highly prized. The value of pure or drinking water is around 2.5 KS per cubic meter.
Most Star Army of Yamatai shuttles carry significant amounts of water as a survival supply.
There are a number of planets that have surfaces mostly covered in water. For this list, 80% or higher is sufficient to be added.
The Yamatai Star Empire tends to favor worlds with large amounts of water.