Tom,
Here's a few articles.
In answer to your questions:
1) From Divernet: "With a tidal range of only 2m between high and low spring tides, the pier is diveable at most stages of the tide."
2) Also from Divernet: "Visibility is usually around 3 to 5m, which is quite acceptable for photographic purposes. A pre-visit phone call to the pier dive shop is always worthwhile and the staff will willingly nip out and check the viz over the handrails. I always call to check the local inshore forecast. The wind that kills viz within a few hours is any sort of easterly."
http://www.divernet.com/travel/piers1199.htm http://www.divernet.com/photog/pier498.htm http://www.divernet.com/travel/1201winter2.htm http://ourworld.compuserve.com/homep...SS/swanage.htm http://www.swanagepiertrust.com/
Give the old pier a try as well (it's just off to the right of the restored pier), there was plenty of fishlife there when I went last year, although it is a bit more tricky to stay within the legs.
I might be interested in coming along - let us know when you're going.
dan.