"mending their nets, tinkering with crab and lobster-pots"

Nets would have been made by hand, by knotting rope.  Mending nets can be a reasonably simple operation or something much larger in scale, as seen below.

Lobster and crab traps are traditionally made from a wooden frame surrounded by rope mesh.  The lobster is lured in with bait and then prevented from exiting, as shown in this underwater video of a modern lobster trap in action.