How can you lose a boat and not know?

Looking at mainland Panay Island from atop the hills of Mararison Island.
Photo from Byajayro

As written in Panay News. 

The speedboat was three to four kilometers from the shoreline of Barangay Aras-Arasan at the time. 

All the passengers, except Dable, were able to hold onto the speedboat when it flipped. According to Jaromay, the speedboat hit Dable on the head.

Tan and Barrentos, on the other hand, swam for more than two hours to the shoreline to seek help until fishermen saw them, Jaromay added.

“Mayor Tan went on board a pump boat and went back to where the boat capsized and rescued his friends and visitors,” the police chief said./PN

I am someone who can't swim and has been to Mararison which is why this news is hitting me a little wrong.