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.
I happened to see the seaman saying something to the captain in a shaky voice.
I found a cute sailor uniform and a sailor cap, so I bought them for my grandchild.
The brilliant actor who played a navy sailor in the play really wow’d the audience.