I liked playing a game where someone threw balls to the bottom of the pool, then we had to pick them all up and line them up on the side of the pool.
この場合、次のような言い方ができますよ。
ーI liked playing a game where someone threw balls to the bottom of the pool, then we had to pick them all up and line them up on the side of the pool.
「誰かがボールをプールの底に投げ、それを回収してプールサイドに一列に並べるゲームが好きだった」
to pick up「拾う・拾い上げる」
to line ... up「...を一列に並べる」
ご参考まで！
I loved the game where we'd dive for balls and line them up on the poolside.
・dive for balls「潜ってボールを拾う（回収する）」
水中のものを拾う動作には dive（潜る）や collect（回収する）を使います。
・line them up「それらを一列に並べる」
I loved the game where we'd dive for balls and line them up on the poolside.
カナダ 
アメリカ合衆国 
