There are 2 divers medical that you will undergo when joining.
1 - In preparation for your PEDA you will be medicalled by the AFCO medical examiner (Diving). Prior to this you should have completed the Health Questionnaire. This pre-PEDA medical is to clear you for the Trial dive you will undergo at the PEDA. It will involve the full military diving medical but minus the chest Xray. This means they will check you breathing and general physical fitness ready to dive. It will include some blood tests.
2. On joining HMS Raleigh to commence your basic training you will undergo a further diving medical as before but including the chest Xray.
Your CA should be able to give you any further info you need on the process of getting medicalled for entry.
Yep, until recently the diving acquaint was a purely physical assessment which could be passed by a fit individual with no diving aptitude, it now includes an "undery watery" bit.
To this end whilst potential divers need have no previous experience, common sense would suggest at the very least you should at least have a 'taster' at diving before PEDA to see if you actually like it.
You also have to give a blood sample, as indicated by SM, so smackheads need not apply.