Now do you mean 40cm from the tip of your spear, or from your eyes? If it's 40 cm of visibility from the spear tip, then it's still clear water (ha ha!), while if it's 40 cm from your eyes, then its' very difficult indeed.
Me I love spearing in very low viz: it's a hunt against shadows, you feel like a ninja, or a shao-lin monk. Fishes are just dark shapes appearing and disappearing while you're surrounded by a mysterious nothingness. Fascinating.
Techniques: there's such a lot to say
-Short guns? Yes (40/60 cm)
-silence and very slow motion
- always current from your back
-remember: you can't see them they can't see you. But they can FEEL you with their sense organs. Only if you don't make too much noise, they won't easily feel your presence.
§Tricks to enhance visibility:
1-find points of referrence on places where fishes may pass around (bright rocks, wreckage, any submerged structure that may "call" fishes). A fish always has a reason to be in a place instead of another. You must avoid "flat" places with no rocks, no vegetation, nothing: fishwill stay away of those places too.
2-fishing in a counter-lìght position, aiming your gun from bottom towards the surface: you stay on the bottom pointing the gun up. You will recognize the dark shape of the fish against the brighter surface (if the sky above is bright enough)
3-fishing along cliffs of bright rocks (marble or so) in two possible positions:
a-in front of the cliff you'll recognize the dark fish's shape against brighter rocks
b-stay clamped on the cliffs in a parallel position, waiting for fish coming in front of you or from back, with gun 45° (degrees) towards open sea.
c-look into holes or cracks between rocks with a torch and, if you see a fish inside, shoot immediately.
Apart of this, the visibility can't be that bad every day. You should learn to forecast underwater visibility from weather, tide, winds, current streams, composition of the bottom (sand/rock/mud etc): this kind of knowledge is necessary.
A good thing, if you spear in areas of frequent lo-viz, is to keep a diary with notes about your habitual fishing spots: everyday you should take note about visibility, presence of fish and related weather-tide conditions. After two round years you'll be able to forecast with good approximation.