And as he was passing alongside the Sea of Galilee, he saw Simon and Andrew (the brother of Simon) casting a net into the lakea—for they were fishermen. Bible see other translations
This is referring to the “Sea of Galilee.”

“the lake.” The “sea” of Galilee is actually quite a small lake, only 7 miles (11.2 km) across and 12 miles (19.3 km) long, and the entire lake can be seen from the escarpments on both the east and west sides. The Greek word thalasso, lake, sea, or ocean, does not really refer to the size of the body of water, and thus has to be translated into the English “lake,” “sea,” or “ocean” by knowing the body of water that is being referred to (see commentary on Matt. 4:18).

“for they were fishermen.” In this case, Peter and Andrew were fishermen. There would be other people who used cast nets to catch fish to eat.

Commentary for: Mark 1:16