Pretty impressive given that Russia should dominate the waters, given that they have their blacksea fleet there and Ukraine has ... like a few boats. When Russia can't even repele an attack at sea, they're really weakening.
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Which raises the question of how
Since the only enemy besides the guards was apparently a single plane, I'd say Russian intelligence didn't get a whiff of this attack and didn't have more planes or ships in position.
So Russian intelligence failed to see this attack, Ukrainian command managed to keep it under wrap and still bring enough forces to see this through and the operation was swift enough that the Russians couldn't reorganize.
Ukraine picked a battle it could win and did. Not much more to ask for, just well planned, well done.
Similiar to how at the start of the war there were many battles Ukraine couldn't win - so they didn't fight them in the first place. That's strategy.
How can they hold it after this?
Good question, but I assume that's opsec.
Probably again acting according to intelligence. When Russia sails the fleet there they're probably just getting out. If they launch a small-scale attack Ukraine may lay a trap.
Yes! Excellent Progress