Navy Sailor
Sailor, Seaman
noun
A water soldier is a soldier that is out to sea. In other words, a navy sailor.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
水兵さんのセーラー服ってどこで買えるんですか？
Where can I buy a sailor's uniform?
「かもめの水兵さん」っていう曲、知ってる？
Do you know the song "The Seagull Sailor"?
あの水兵が、船長にふるえ声で何かを話しているのを見てしまったんだ。
I happened to see the seaman saying something to the captain in a shaky voice.