You weren't there, don't know what happened (other than what has been posted here) and you want someone arrested for pushing? Wow.
"He-said she-said" is NEVER a good enough reason to arrest someone (even if it happens regularly).
The underwater world is full is miscommunication -- there's no reason to get the POLICE involved, especially if there is no photo or video evidence of the alleged "assault".
I'm just going through this thread now so I'll probably respond to a couple of posts as I go.
As far as 'assault' as a crime goes this one is pretty thin but getting the police involved might not be a bad idea for two reasons.
One, because the best way to fight a bully is to stand up to them and making sure that the instructor and the shop are visited by police and that there is a police report generated (even if charges are not filed) gives a very clear message that you (I mean, the instructor/shop/student involved) will not stand for this kind of behaviour. If it happens more than once then the shop may in fact, end up having to fire this employee for the protection of their own business, which is what you want. You want to show them, especially the instructor, that there will (probably) be consequences if it happens again.
Secondly, because this behaviour is creating potentially very dangerous situations under water and *IF* it escalates to the point that it becomes necessary to "go legal" with them, or (heaven forbid) the instructor causes an accident, then the police report(s) will serve as evidence in the case.
Oh, and btw, it's not hearsay if the people directly involved file the complaint.
R..