The first one to know about their prey arriving was Will. He felt a large shape moving in the sea kilometers away. Just the sheer displacement of water was enough for him to have an idea about its location.
They quickly sent away Lucy to get Sera and Bea, then when they finally arrived it was time for the confrontation.
Before setting out on this expedition they knew that this battle is going to be different, because within the water only Sarah and Will can fight effectively. Faith and Lucy can also cause issues for the monster, but no one else really... Except Vicky, her attacks are effective against anything, but no very powerful.
The plan was to keep the beast near the surface, so everyone can at least have some way of attacking. This is going to be Will's and Sarah's job, delivering attacks while distracting.