Liberals think that if you put enough intermediate steps and performative civility over the direct violence part that it stops being violence. So because the ships aren't being fired on with rockets or cannons if they try to go to Cuba, that's not violence. Instead if someone breaks the terms of the embargo while in the US they'll be arrested (which is violent, but liberals don't think it is), if a ship docks in Cuba it's barred from docking in the US for a long period of time (and if it tried it would presumably be impounded and its crew arrested, all of which involve violence or the threat of violence), if a company outside the US violates the embargo it gets blacklisted or sanctioned by the US (all of which is ultimately enforced by the US ability to enact violence), etc.

The capacity for violence is the foundational force of state and geopolitical hegemony, and every layer of power on top of it is backed up by the threat of violence or carried out by people who are under the threat of violence if they don't comply. Even things like fiat currency are basically just operating on the hegemonic violence standard: liberals think fiat currency runs on magical thinking and vibes, libertarians think it's fake altogether, and only occasionally will you see some honest admission that the US dollar (for example) is actually based on America's capacity to enact violence globally and domestically.