In general, what is generally referred to as "beach detecting" is not really one of the concerns of this blog, searching loose tide-borne deposits for modern coin and jewellery losses seems like a jolly good use of a metal detector, involving fresh air exercise, the thrill of the search and discovery, develops an ability to 'read' the conditions to pinpoint likely search areas, probably brings you into contact with all sorts of interesting people who stop and ask what you are doing, and so on. And all without doing any damage to the archaeological record. Great, go for it. So over on the new "The Responsible Detectorist blog ("The occasional ramblings of a part time metal detectorist in West Sussex, trying to promote responsible dectectoring") is an interesting post "What is responsible detecting on a beach?".
I'd add to Steve Broom's very pertinent points that at some points around the coast (including, I believe West Sussex) there are sensitive and stratified archaeological sites exposed on the surface of the foreshore, and below high tide-mark. These include sites that were originally on dry land and the land surface has sunk in relation to the sea level (the Thames estuary for example is in a huge syncline). They will include sites originally at high tide mark (salt pans and extraction workshops, my archaeological first love), and sites originally in the water now rising out (fish traps etc). Obviously simply hoiking material out of them for private collection is not a "responsible" way to treat such sites. However not all such sites encountered in artefact hunting will be known (or they may have changed form since last visited by an archaeological search team). So, it's worth recording and reporting anything that looks archaeological to the local archaeology services (assuming local government cuts have left one!). A good way to do this might be a series of (time-stamped) digital photos, linked to GPS co-ordintes, best to record that in a notebook, dont forget shots from a distance allowing the siting and setting to be recorded. I think I'd try bot to touch it, get expert advice before trying to alter its state. Perhaps try get the (probably otherwise very overworked and understaffed) archaeology team to go out to the site with you and show you what to look out for and how they want you to deal with it (if at all). It is also worth trying to verify the state of old sites known from archival records. Oh, and do please keep a lookout for briquetage. Brilliant stuff.