For example, if it is hot out and the line for the "whites only" water fountain is really long, but the negro water fountain is open. Is there a penalty for just using the negro fountain?
White people didn't face legal repercussions for using "colored" fountains. There would have been a chance of social derision from the White community, due to the association with African Americans, or the significantly lower quality of "Colored" specific areas but nothing illegal would have happened if a white man used a black restroom or water fountain.
To clarify: It's possible that this would have been illegal in some municipality, but I haven't read anything on it.