There is ample precendent for impounding the assets of hostile nations. The Soviets did it to Germany in WW2, so they cannot really claim that they are opposed to that practice.
The only reason why this seizure of russian money in Belgium might be a bad idea is reciprocity. Russia would of course then try to seize European assets in Russia.
And regarding ships, prize law is still internationally accepted and in effect. Ukraine can offer prize letters to privateers or foreign navies, allowing the seizure of Russian ships. Or they can seize ships themselves. When those ships are then in a Ukrainian or allied harbor, a Ukrainian admirality court then assigns ownership of the vessel and all goods to the ones who brought it up. https://en.wikipedia.org/wiki/Prize_(law)
The only reason why this seizure of russian money in Belgium might be a bad idea is reciprocity. Russia would of course then try to seize European assets in Russia.
And regarding ships, prize law is still internationally accepted and in effect. Ukraine can offer prize letters to privateers or foreign navies, allowing the seizure of Russian ships. Or they can seize ships themselves. When those ships are then in a Ukrainian or allied harbor, a Ukrainian admirality court then assigns ownership of the vessel and all goods to the ones who brought it up. https://en.wikipedia.org/wiki/Prize_(law)