John Gill Commentary Mark 1:16

"And passing along by the sea of Galilee, he saw Simon and Andrew the brother of Simon casting a net in the sea; for they were fishers." — Mark 1:16 (ASV)

Now as he walked by the sea of Galilee
The same with the sea of Tiberias, (John 6:1) ,

he saw Simon :
whose surname was Peter, the son of Jonas:

and Andrew his brother ;
the brother of Simon,

casting a net into the sea ;
of Galilee, in order to catch fish:

for they were fishers :
by occupation, this was their trade and business, by which they got their livelihood; (See Gill on Matthew 4:18).