Ships could also proceed in the 12-mile territorial limit of romania and bulgaria, then reflag under romanian, bulgarian, the turkish flags. Any attack once out of ukrainian waters would be an article 5 violation.

Also apparently there are hundreds of ships. Does russia have hundreds of submarines and backfire bombers ready to be destroyed in the act of sinking a 10k ton romanian flagged ancient grain hauler?

I's suspect that aside from bluster, mines, drones, and sabotage would be the biggest threats

And finally, this all rests on https://en.m.wikipedia.org/wiki/Montreux_Convention_Regarding_the_Regime_of_the_Straits

It was challenged by the russians in the past and is one reason Turkey is in NATO. With Russia violating all rules of conduct recently, maybe it's time to replace this with the following rules:

1. all military traffic forbidden - for russia

2. all military traffic allowed - for NATO and other black sea states

Especially when combined with eventually evicting russia from syrian naval facilities, it would really improve security in the entire region.