[spearheads]

Well i dont know about your area of diving but lots of spearo are beginning to catch fish mainly mullet down this way.
As for your weight belt you should be fine as long as you are not over weighted, you should be neutrally buoyant at 10 meters and should not sink on the surface if you do you are overweight.
Remember you got to relax in the water to be able to hold your breath.
And yes do take a knife you could get caught in old fishing line or a lost gill net.
You shouldn't really be loading you gun out of water, i will try to find the link that shows how to load a spear gun.
Good luck
cant find the video only this pic, i dont load my gun like this i place the loading butt on the buckle of my belt then grab the rubbers and pull them back until the wishbone locks in place.