dnd 5e – Did I rule this correctly? Readied actions and turn order

The monster my players were fighting acted first in the intitative order. It dove into the lake and out of sight. They all readied actions to attack when it revealed itself. On its next turn it attacked, triggering all of their readied actions, so it got off its attack, but then they all attacked him. We all got really confused as to whose turn it now was, the monster again or my players. I ruled that it was the monster but now that seems wrong.