I don't think that there's any realistic chance that Ukraine can make use of ships in the Black Sea. Russia built their military to contest the US in the Pacific -- they've got a lot of long-range anti-ship weapons. That surplus capacity is why they've been blowing anti-ship missiles on land attack. I'd be pretty confident that Russia can keep a Ukrainian warship from surviving in the Black Sea. Where Ukraine's pulling off naval attacks, it's using either small, very-low-profile boats or even-lower-profile, mostly-submerged USVs. Russia apparently doesn't have the sensor capability to reliably pick those up (and I imagine that Ukrainian strikes on radars probably also complicate that).
I have wondered about maybe Ukraine using larger UUVs that surface to launch a weapon. Such a UUV would have to be something that could be transported on a trailer, so there are some size limitations. But it might permit for a more-capable platform than the small USVs that are currently being used.
I don't know what kind of anti-submarine-warfare tools Russia has available in the Black Sea, but if they aren't able to detect the existing USVs, I would assume that they aren't going to be doing better with UUVs.
EDIT: There's a reference to a Ukrainian UUV project in progress here; it says that Russia is improving their ability to detect the existing Ukrainian USVs, so UUVs are becoming more important.