How did Jacob knew that Bella is in the water/drowning?

Well the pack was tracking victoria that day. She jumped into the water before they could catch her. Jacob got worried because he knew how much time Bella spent at the समुद्र तट in la push, so he went to the समुद्र तट to get her before Victoria could. When he got there, Bella was already drownding so he save her.

they were tracking victoria so they followed ger. when they realized bout bella he saved her just in time