Mosses are non-vascular plants which do not have water conductive tissue made up of xylem and phloem. They have simpler systems for conducting food and water. They require water for completing their life cycle due to following reasons:
1) They live in damp and marshy areas on land and therefore cannot survive in dry regions.
2) They require water to complete their life cycle through sexual reproduction because the sperm of mosses (antherozoids) are flagellate and need water to swim to the eggs.