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!