Putin acted like a stupid kid messing with those Ukraine boats. A Navy isn’t a toy- be an adult at least.
Now he's seized those naval ships to emphasize that he stole that land fair and square and he controls that water and Ukraine better not try to say otherwise or he'll pop them like pimple. That's it. It's a bullying move.
In the end, the port of Sevastopol is never going back to Ukraine, at least not without a perpetual lease or something like it being given to Russia. That's what this is all about. Obama looked the other way when Russia seized it in 2014, now short of military intervention there's isn't much that can be done to get them out.