what sea is south of Ukraine

The sea you're looking for is the Black Sea. I've sailed those waters plenty of times over the last few years, and it's a pretty distinct region once you get familiar with the currents and the ports along the Ukrainian coast like Odesa.

It's honestly a busy stretch of water. Aside from the Black Sea, you've also got the Sea of Azov tucked away a bit further to the northeast, which connects through the Kerch Strait. Both of those are pretty important for shipping and trade in that part of the world, though the weather can get quite choppy depending on the season.