I see no significant difference.
Are these sentences correct:
1) Tell me when you are entering the pool.
2) Tell me when you enter the pool.
I think '1' could mean two things:
a) When you are entering the pool, tell me that you are entering the pool.
b) Tell me when you will be going to enter the pool.
I think '2' would in general only mean 'a', but it could also be used if 'entering the pool' was a habitual action (for example in this sentence: 'Tell when you enter the pool every day.')
Would you say that is correct?
I think b) is a bit dodgy as an interpretation of 1).