I agree, the careful harvest of any resource is important in maintaining that resource. Most local authorities put limits such as hunting seasons, weight/size limits, numbers per day etc. All those are part of managing those resources and should be seen as such. I don't see that Divers taking lobsters or fish is signifcantly different from a hunter taking a deer or a fisherman taking a salmon. My view, if you're going to kill it, you should be planning on eating it! Killing just for sport or a trophy, whether it be with a bow, rifle or speargun, is not something I would condone.
One difference from a purely sporting standpoint, when fishing, one can usually practise catch and release. Hunting, spearfishing etc. obviously does not give that option.