What were James and John doing in their boat when Jesus called them in Mark 1?

Answer: Mending their nets

James and John were mending their nets; they left their father Zebedee in the boat and followed Jesus.

“Going on a little farther, He saw James son of Zebedee and his brother John. They were in a boat, mending their nets. Immediately Jesus called them, and they left their father Zebedee in the boat with the hired men and followed Him.”— Mark 1:19-20 (BSB)