Question

what is the old oligarchs main idea when he says it is the steersmen and the shipwrights who have brought the city state power?

Answer 1

By showing the citys rules for boating.
The existence of rules for boating indicates that the actions that initiated by both the steersman and the shipwrights are regulated under a national regulation that would trump local regulation. This could be translated to them bringing the state power for the city.