John Chrysostom

This was a sign of leisure, but according to Matthew He finds them mending their nets. For so great was their poverty, that they patched up their old nets, not being able to buy new ones.

Catena Aurea by Aquinas ↗

Bede

But the fishermen had disembarked and were washing their nets. The fishermen are the teachers of the Church, who, having caught us in the net of faith and raised us from the depths to the moon, bring us to the land of the living like fish to the shore. For just as the nets are entwined, so are the words of the preachers, which do not lose those they have caught in faith. Hence, the nets are called "retenia" as if they are retaining. …

On the Gospel of Luke ↗

Bonaventure

Second, there is added the opportunity of a boat lifting him up upon the water, when it is added: And he saw two boats, opportune, namely, for carrying, both because they were near: on account of which he says: Standing by the lake; and also because they were idle or empty; whence he also adds: But the fishermen had gone out and were washing their nets: and thus they were opportune for service. …

Commentary on Luke, Chapter 5 ↗