Welcome! I hope you enjoy it here, I do
I don't know how to help, but my pup has learned how to jump in and out of the bath as well.
Have you tried teaching him 'sit and stay' while you're in the shower? As Dexter likes to follow me everywhere I bring him a toy in the bathroom so he can play in peace and not try and jump in with me lol
Also, he has met all my neighbours in the flat I live in and he still growls at them...maybe he thinks we own the whole building, I don't know.