The US claims that Russia is planning to attack civilian ships
in the Black sea and
claim that Ukraine's mines blew them up.
Also that Russia has planted more mines.
Ukraine can legitimately retaliate against Putin's threats to attack
civilian ships sailing to and from Ukraine, by attacking ships sailing
to Russian ports. Especially the main Russian oil export port,
Novorossiysk. To avoid polluting the shore, it should attack tankers
traveling empty into Novorossiysk. To maintain better relations with
other countries, it should attack Russian-owned or Russian-flagged
ships, but not make a commitment to limit attacks to them.
This would do enormous economic harm to Russia, and I expect it would
convince Putin to reestablish the Ukraine food export deal.
Of course, there may be reasonable things for Ukraine's supporters
to do to facilitate export of food from Russia.