A heavy rain can wash silt & mud into rivers & streams, sometimes reducing visibility for days afterwards. In spring, we often get a bloom (plankton?) that turns the water milky for days or weeks at a time. You don't need or necessarily want perfect visibility to spearfish though - but you do need to be able to see beyond the end of your spear. In gin-clear conditions, the fish will be able to see you better too
Big low frequency swells make me sea sick but there is often some swell in the sea. I prefer it to be flat as a mill pond but that is fairly rare (but had a great evening spearing in Devon under such conditions). A little higher frequency chop is tolerable & normal too but I'd rather not have it as it too can make me sea sick.
Calm weather for several days beforehand and in the forecast for the day is usually a good sign. Many bays have their own little micro-climates though. Swearing at rubbish BBC weather forecasts has become part of spearfishing for me - didn't know whether to laugh or cry when I heard that (1) the BBC weather forecasters recently won some prize for accuracy (like the day I had 4" of snow last year & their forecast continued to show none even after it had fallen and while more was still falling), they are
[email protected] - the American forecasters are FAR more accurate and consistently so - and (2) the BBC weather forecaster just been given big bonuses for their accuracy!

:waterwork

:waterwork Windmap & ITV are generally more accurate (& far less tedious).