Diving
noun, する verb
To conceal yourself in water, you'd better start diving and get under the water where it can conceal you!
せんすい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
うちの妹は今年の夏に潜水士免許を取得したんだよ。
My sister got her diver’s license this summer.