let down your nets for a catch: Peter was naturally unsure when a teacher told fishermen how to catch fish. It seemed strange for three reasons:
When the sun rose, the fish went down to the bottom of the lake. This is why they usually fished at night.
They used a heavy net that hung from the side of the boat, held by two men (or stretched between two boats), so they could not fish in deep water.
They had just fished all night and caught nothing, so it now seemed even less likely they would catch anything.