We thought we could all get on the duck shaped floaty at the same time, but we all fell into the water at once.
この場合、次のような言い方ができますよ。
ーWe thought we could all get on the duck shaped floaty at the same time, but we all fell into the water at once.
「アヒル型の浮き輪にみんな同時に乗れると思ったけど、みんな一気に水に落ちた」
to get on the duck shaped floaty で「アヒル型の浮き輪に乗る」
to fell into the water で「水に落ちる」
ご参考まで!