Freya DH and I did the SSI course about 6 years ago now (a PADI rival LOL) but either way diving is fantastic.

If you head north you can do dives on the Reef without having completed a course (they make sure you don't go below 5-6 metres so there is no chance of injury). We are in Melbourne and I think you can do "try dives" with some companies but it is not as common as in Qld.

As for getting left behind - that is what your dive buddy is for! You always dive in pairs (at a minimum) so you can look out for eachother.

And as for sharks, it is VERY rare for a diver who is not spear fishing to be taken by a shark. You can avoid areas where sharks are common (I would NEVER dive off Sth Australian waters where the abalone divers are often taken for example) and if you are really freaked you can buy these "anti-shark" devices which keep them at bay.

Scuba diving is truly amazing - I am just sorry I haven't been able to do it more since having kids. It is like flying Flying on another planet!