Also, Ukraine will demine its ports, giving Russians some good opportunities for amphibious landings.
An opposed amphibious landing was among the most difficult military operations conducted in WWII, which is why the largest naval armada in history still relied heavily on misdirection (successful, the bulk of the German mobile forces were at Calais on June 6, 1944) to drive as many enemy forces somewhere else as possible.
It is far harder now. Added to the advantages of the defeneder are long-range shipkiller missiles such as the Neptune and Harpoon systems that Ukraine has already killed major fleet assets with, as well as ATGMs that are more than capable of turning a landing boat or hovercraft into a blood-sacrifice to Posiedon in an instant. Countering that requires massive naval firepower (which Russia may never have had, and certainly doesn't now that
Moskva is at the bottom of the sea), overwhelming air superiority (which Russia doesn't have - Ukranian-operated SAM systems and the stubborn air force has made the sky neutral at worst), or complete and total surprise (Which Russia doesn't have because Uncle Sam is whispering Russia's every move to Ukraine's ears).
An amphibious assault sounds scary, but if Russia ever tried it the odds are overwhelming that the only result would be the annihilation of Russian naval infantry in the region, and possibly the end of the Russian Navy in the Black Sea.
This agreement is no threat to Ukraine. And it goes a long way toward blunting one of Russia's few remaining soft-power weapons - if that grain gets where it is going, Russia won't be able to point at starvation in the Middle East and Africa and blame it on Ukraine.
In other news, I've seen some reports that Russian troops in the Kherson region have been cut off and are requesting evacuation corridors, which Ukraine has flatly (and correctly) refused.