There's a competitive 'skins' wave, (swum roughly under ASA type rules, so leave the budgie smugglers at home if you can fellas) and there are several waves for those in suits. The wave you are in is dependent upon what time you register rather than ability.
There are some very fast swimmers in all waves, as well as those out for a gentle float, so regardless of your swim speed you'll be in good company. The local open water swim group, the Chalkwell Redcaps always turn out in force and they are a pretty friendly bunch with skins, channel swimmers and brand spanking newbies among their number. Their Chairman, Iain Keenan was instrumental in setting the current course that goes through Southend Pier (the longest in the world no less).
What's not to like? A fun, friendly event, long enough to make it worth travelling to, the home of the Rossi ice cream and making loads of money for the local Hospice.
Go on, give it a go. You won't be disappointed.