You can fish all around the UK except in conservation areas and designated bass breading areas, you also have to avoid harbours and mariners (the inside of them that is) and protected wreck sites where exclusions zones may be in exsitstence. Visability varies from area to area, South, South-West coast, Wales and Scotland are all recognised areas for spearfishing. I only have a working knowlwedge of the south coast from bexhill to Wittering, and limited experience of the south west, I have also dived in the channel islands. Visbility is dependent on wind direction and when an east wind blows down here on the south coast on most occassions you forget it. Visibility however can be very good, before Christmas we had a periond of calm and the visibility was up to 15m+ on other occasions it can be down to zero. 5m-7m in the summer might be considered very good. A 5mm wet suit is needed for the beginning of the year although some are now using 7mm jackets to get longer in the water, 3mm is fine during the summer months. 75cm guns are the most common were i fish increasing to 90cm or double strung 75's when the viz is good or you are after bream.
If you want to know anything more specific post some more questions
