Well, one ship can be boarded and captured even at heavy loss, but a city-state with cannon technology now can enforce its will anywhere within cannon range by threatening devastating bombardment of coastal cities. Hence the term gunboat diplomacy.
Moreover, in terms of the balance of power, it’s a bit like what happened when the Monitor and the Merrimack gave birth to the era of the ironclad; as the London Times put it “whereas [previously] we had available for immediate purposes one-hundred and forty-nine first class warships, we now have two.” A cannon-carrying ship could manuever to maintain range against standard war galleys which needed to close to ram or board their enemies – as the Ottomans found out when they fought the Portugese for control of the Indian Ocean in the 16th century.
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