John Gill Commentary Mark 1:19

"And going on a little further, he saw James the [son] of Zebedee, and John his brother, who also were in the boat mending the nets." — Mark 1:19 (ASV)

And when he had gone a little further thence
From the place where Simon and Andrew were casting their nets, though still by the sea side;

he saw James the son of Zebedee, and John his brother .
The former was he whom afterwards Herod killed with the sword, and the latter the beloved disciple; these were also fishermen:

who also were in the ship mending their nets :
as the other two disciples were on board the ship, casting their nets into the sea to catch fish; these were also in a ship, repairing their nets, in order to use them the same way, and for the same purpose; (See Gill on Matthew 4:21).