I got into spearfishing back in the beginging of the summer. Most of our hunting has been around Portland Bill with some great trips had...
I dont expect anyone to tell me any particular marks but is the spearing East of Portland towards Kimmeridge and Durdle Door worth looking into???
The Smedmore Estate have banned spearguns at Kimmeridge, according to the Marine Wardens - if you are seen with one down there you will be asked to leave. They are quite precious about their bay, as far as spearfishing is concerned, especially since someone lost a loaded gun on the bottom, in an area that divers and young snorkellers often frequent (it is still down there, somewhere, loaded and ready to strike).
Apart from it being a voluntary 'no-take zone', the local shale means that Kimmeridge is often troubled by poor visibility (which is further exacerbated by the fact that it produces surf if there is any kind of swell about).
Having said all that, Kimmeridge is a good spot to launch a boat for the marks further west of the Bay (all the way up to Lulworth Cove, which is another launch spot for the marks even further west, towards Weymouth), although check that the military are not using the ranges first, or you might have a wasted journey.
Best to leave Kimmeridge Bay itself alone: there are many great (and rarely fished) spots between there and Weymouth - check out marine charts for likely fishy areas and Ordnance Survey maps for access by foot (if you don't have a boat then quite often a bit of a walk will deliver you to a lovely spot that you will have all to your self).
The classic, well known and more easily accessible marks are at the areas around Ringstead, Durdle Door and Lulworth.
Good luck.