I've put together the following which covers some of the open water (or at least open air!) meets and challenges which you may be interested in which are in the South of England. Please be aware, other swims are available, if you come across any, let me know (I'm not on Facebook!) and I'll add them to the list and recirculate nearer the summer.
 
2Swim4Life 2011, Guildford Lido, Surrey
30-31st April 2011
http://www.2swim4life.co.uk/
24 miles (1 mile an hour, every hour, for 24 hours)
Wetsuit or non-wetsuit, but body suits are being encouraged for the night hours
Raising money for Help for Heroes, this has to be one of the most demanding swims you can do in the pool. First undertaken in 2008, you swim a mile starting at 9 am. At 10 am, you start another and so on until 8am the following morning when you hopefully swim your 24th and final mile.  This year's event will be held at the 50m Guildford Lido which is open air. There is also the option of doing 12 miles (through the night).
If you are interested in taking this challenge on and have any questions, let me know as I swam the first edition of this event in 2008. Enter soon though, as spaces are limited and are filling up.
 
Fairway Buoy Swim,
Exmouth, Devon, 15th May 2011
www.exmouthswimming.org
4.75 miles (Sea, tidally assisted)
Non-wetsuit only
Entries close 16th April 2011 and only 32 places for this tough swim, held under BLDSA rules. It's so hard and cold (perhaps only 11 degrees), Tim has to sign your form to say you can manage it!
 
BLDSA
http://www.bldsa.org.uk/
Non-wetsuit and some specific rules regarding costumes - please see BLDSA site for details
British Long Distance Swimming Association (BLDSA) events are testing - no wetsuits, long distance and often cold water. Generally, events are open only to BLDSA members, which costs around £35 to join annually. Look on the website for a list of events, ranging from short swims to the monster Windermere Two Way race weighing in at 21 miles!
 
F3 events Evening Open Water Swim Series, Dorney Lake, Windsor
18th May, 15th June, 13th July and 17th August
http://masters.oxfordswim.com/ (Accessible from the Entry forms page)
750m or 1.5km events
Another in the list of open water swim series. Entry information can be found through the link on our COSC Masters website. Just go to the 'Entry forms' page.
 
Endurancelife Wild Swim Series
http://www.endurancelife.com/event.asp?series=48
Erme Estuary, Devon, 8th May 2011 (3 miles)
Helford Estuary, Cornwall, 11th June 2011 (4 miles)
Dart Estuary, Devon, 17th June 2011 (5.2 miles)
Wetsuit only
Many thanks to Liz for forwarding this onto me. A series of point-to-point swims in some of England's most picturesque estuaries and supporting the Marine Conservation Society. The organisers will do a discount for group entries, so if you are thinking of entering, get together with some others.
 
Open Water Swim Series, Little Marlow, Bucks
Series of 5 events: 26th May, 16th June, 8th July, 29th July and 12th August (dates are tbc)
http://www.openwaterswim.co.uk/
750m, 1500m and 3.8k races available at each event
Wetsuit and non-wetsuit
This series got even more popular than Thursday night training for us last year!  These dates are to be confirmed, but the events are well organised and the lake is warm, clean and clear. Come along and you might swim against an Olympic legend (Adrian Moorhouse swam in one race last year) and leave with more kiwi flavoured Powerbars than you know what to do with!
 
Royal Windsor Open Water River Swim, Windsor
5th June 2011
http://www.f3events.co.uk/index.php?option=com_jevents&view=cat&task=cat.listevents&Itemid=15
950m, 1.9km and 3.8km events
 
RNLI Long Swim Series 2011
http://www.votwo.co.uk/votwo2007/events-and-adventure/race_view.php?v=2&c=1&i=157
South Cerney, Glos, 12th June 2011
Box End, Bedfordshire, 17th July 2011
Bournemouth (sea swim), 14th August 2011 (date TBC)
Dorney Lake, 25th September 2011
5th venue and date TBC
750m, 1 mile and 3.8k races available at each event
Wetsuit (non-wetsuit welcome, but no separate category)
The VoTwo series is now sponsored by the RNLI and will no doubt continue to be popular.  Races consist of laps of a course and are well organised with results available as you stumble out the lake and over the line. The Bournemouth swim is in the sea, starting from the beach.
 
Seahorse Swim (organised by East Dorset Open Water Swimming Club)
Studland Beach, Dorset, 26 June 2011
http://www.edowsc.org/ed/archives/2761
3.8km (Sea)
Wetsuit or non-wetsuit
Last year was the first year for the Seahorse Swim, but all 150 places filled up quickly and entries are open once again for the 2011 edition. A 3.8km swim in the sea off one of the most picturesque beaches in England, with the bonus of the entry fee helping charities that aid seahorses.
 
British Gas Great London Swim, Docklands, London
2nd July 2011
http://www.greatswim.org/Events/British-Gas-Great-London-Swim/Default.aspx
1.5km
Wetsuit
Up to 5000 did this event last year (they set off in waves!) and all abilities from beginners to Olympic medallists take part. Other swims in the Great Swim series are available from here http://www.greatswim.org/Events/Default.aspx
 
Big Marlow River Swim
16th July 2011
http://www.f3events.co.uk/index.php?option=com_jevents&task=icalrepeat.detail&evid=33&Itemid=15
750m, 1.5km and 3km events
 
British Heart Foundation Swims
17th July: Bournemouth (1.4 miles pier to pier) and 7th August: Weymouth (1 mile)
http://www.bhf.org.uk/get-involved/events/swims.aspx
Wetsuit or non-wetsuit
Take a dip and raise some money for the British Heart Foundation. Be warned though, these are 'mass participation' events, so expected plenty of company and no prizes for winning (but that's not the point, isn't it...)
 
British Gas ASA 3km Open Water Masters Championship 2011, Rother Valley Country Park, Sheffield
24th July 2011
http://www.swimming.org/britishswimming/calendar/event/british-gas-asa-3km-open-water-masters-championshisps-2011
Non-wetsuit
Make it two trips to Sheffield for national championships this year.
 
RLSS Poole Lifeguard Brownsea Island Swim, Poole, Dorset
31st July 2011
http://www.rlss-poole.org.uk/brownsea.htm
4.6 miles (Sea)
Wetsuit and non-wetsuit classes (non-wetsuit does not allow legskins or arm coverage)
A popular swim around the National Trust-owned Brownsea Island in Poole Harbour. As the island is in Poole Harbour, water conditions are generally flatter than the open sea and maybe a degree or two warmer. Water temperature is likely to be around 16-17 degrees. At 4.6 miles (with the distinct possibility of being further given you navigate yourself) it is a long swim, but the second half is generally tide-assisted and in warmer water.
Entry form will be available from the website in late February. If you are interested, be quick to enter once the form is available, all 200 places were filled last year within a couple of weeks.
 
Big South Swim 2011, Dorney Lake, Windsor
20th August 2011
http://www.f3events.co.uk/index.php?option=com_jevents&task=icalrepeat.detail&evid=32&Itemid=15
950m, 1.5km, 3.8km and 5km
Wetsuit only
 
Henley on Thames River Swim, Shiplake College, Henley
3rd September 2011
http://www.f3events.co.uk/index.php?option=com_jevents&task=icalrepeat.detail&evid=31&Itemid=15
750m, 1.5km and 3km