Although there are quite a few nice beaches along the Black Sea coastline, it's not particularly suitable for a vacation that's beach oriented.
The weather will definitely be fine, you shouldn't worry about the temperature. Nor should you worry about the water being too cold either. However there are still a couple of peculiarities that make the northern parts not usually preferred for tourists looking for a beach oriented vacation:
The humidity might be overwhelming in hot days if you aren't used to it, and depending on the particular city or town you visit, the rainfall might be a little too frequent than you'd like... Especially on the eastern parts. And as downhald put it, the Black Sea coasts usually deepen very quickly, which is fine, but for people who are into enjoying the shallow waters this will definitely be a turn-off. Plus, when there are strong winds, the quickly deepening seashore combined with the characteristically high and strong waves make it quite dangerous to swim. So you'll have to stay out of water if there are strong winds.
All those factors make the southern shores the preferred travel location for people who want to spend their vacations in beaches. If that's the case, you should stick to the southern or western shores.
That being said, the northern coastline has an amazing natural beauty if you're looking for a vacation combining both beaches and nature trips, cave exploration, historical sightseeing, etc. In that case, you'll definitely love it.