Many dive centres set up a course called something like “dates TBA”, as below:
And then book customers on to this course.
When a customer then selects their dates you can move the customer from this course by going into the course, highlighting the customer name and then right click on your mouse.
One of the options you will see is Move to Different Course, as below:
You then select the course that the customer wants to enrol on.
This action will take all payment details over to this course automatically so you will see how much, if any, is still to be paid, for instance.