Credo is needed to navigate ships across deep waters. As long as the character is at the helm:
L1 - Can navigate knarrs
L2 - Reduced ship damage over very deep water
L3 - Ships sail 10% faster
L4 - Whaling cause less damage to a ship (otherwise make knarrs ans snekkjas significantly more vulnerable to whales and orcas)
L5 - Can pass very deep water (boat speed is low and out of control otherwise over very deep water)