The wikipedia page and linked references for the inspiration for the fictional Nucky Thompson: https://en.wikipedia.org/wiki/Enoch_L._Johnson
https://en.wikipedia.org/wiki/Boardwalk_Empire#Cast_and_char...
The references beyond that are quite interesting as well. I always felt that the character Nelson Van Alden (also George Mueller) was a reference to Baby Face Nelson, who was known also as George Nelson.
If you enjoyed Boardwalk, you should also like Peaky Blinders.
Being set in the same era, there are interesting crossovers like when the Peakies get involved in selling whiskey to the US.
Many of these characters are also loosly based on real people like Tom Hardy's Alfie Solomon who steals many a scene much like Stephen Graham's Capone.