Tumbling_Dice,
To clear things up, it is called Ship Team Diver Course (STD for short) not the Diving Officer Course, and it is 6 weeks in duration. Officer's or NCM's over the rank of MS/MCpl who go on the STD course will also do an extra Supervisor portion that will allow them to be Diving Supervisor's. Once you join your ship you may become the Ship's Diving Officer, as a Secondary Duty. Similar to Boarding Party Officer, which is also a secondary duty.
As a qualified STD, you will be allowed to wear the STD dolphins, which is different from the Clearance Diver dolphins.
Clearance Diving Officer (CLDO) is a MARS D-level (20 month long course). For example there are a few Navigating Officer's that have become Clearance Diving Officers as their second D-level. Actually, until recently most CLDO's have had MARS D-level prior to volunteering for the Clearance Diver Prelim.
I hope that clears things up,
Cheers J.