Is Killer Bee still alive?
Yes. Killer B is still alive.

If you watch the full movie, including the credits, you'll see Gyūki (8-Tails) pulling him out of the water. As also mentioned on Wikia: Killer B: Boruto The Movie


  After they finished extracting Gyūki's chakra and throwing an unconscious B into a river to let him drown, Momoshiki commented that he was unimpressed with the tailed beast's power, as he could only produce a few small red pills with it. 


The little size of chakra pills foreshadow that the attempted extraction of Gyūki failed and hence Killer B lived. This is confirmed during the credits.