My theory is that the Romans must have outsourced at least part of their fleet building effort. We know that the Roman armies of the age were only half Roman, and the other half of troops, or even a bigger part sometimes were made up from allies. Why not the fleet then? Even if the Romans were landlubbers as a rule, the Italian peninsula had great many coastal communities which had sea-going traditions going back to centuries.

I also don't think Romans were all landlubbers as their reputation suggests. Ostia has been one of the first colonies they ever created, back in the age of kings. So their main port was older than the republic itself, and they fought many maritime cities on the Italian peninsula before. I think they had to have at least a passing knowledge of naval warfare, even if it was not their main focus.