And going on from thence - From the place where he had found Peter and Andrew, Matthew 4:18.
Saw two other brothers - They were men engaged in the same employment, as it is probable that there were many such in the neighborhood of the lake.
In a ship - A small vessel. In fact, it was little more, probably, than a sail-boat.
Mending their nets - A very common employment when they were not actually engaged in fishing.