RULE NO 1 OF ANY DIVING:
NEVER DIVE ALONE! (but you knew that, right!)
Doubles... not being a spearo, I can't really help you - but I have snorkelled extensively at Mabibi and Scuba Dived off 9 mile Reef. One thing you need to be seriously careful of a Shore entry at Mabibi - there is a gap opening in the Reef about 80-100m out, which becomes a seriously dangerous rip as the tide turns to go out... have seen people get swept out to sea through that gap on a rushing current.
The water at high tide in the Bay at Mabibi is only about 5-7m deep, and depending on how "up" the sea is, is normally quite clear. Past the Reef, it gets steadily deeper, but I have never ventured out through that gap for fear of not being able to make it back...
Other spots... Sodwana has plenty offshore spots to Spear in. Further South from Sodwana is Adlams Reef and Kingfisher Bay - both known great diving spots... but they are 8km and 20 km south of Jesser point.