Me and Mrs I nearly went to Mar Menor last year (when we were looking for somewhere to get a week's late season dinghy sailing). In the end we couldn't find a school (or anywhere else to hire a boat from that late in the season) so ended up going elsewhere.
If you do go there, Stiofan, I'd appreciate it if you could look out for sailing info and in particular any places that hire dinghys.
Hmm, that's interesting as that site indicates in the activities section that they hire out boats, windsurf etc. and I've read elsewhere that it's a top place for watersports, hence my interest. - What month didn't you go then?
I last visited the area off season (November 2003) and stayed in Lo Pagan on the north side of the Mar Menor close to the San Pedro del Pinatar regional park - flamingos on the salt flats etc. (Stayed at the Barcelo Lodomar - but didn't take up the offer of saltwater/mud "thalassotherapy" spa treatment! ) There are some much quieter beaches at the resorts on this side.
IMO the La Manga strip in high season is like Benidorm - except that it is full of Spaniards rather than Brits. There continues to be massive multi-storey redevelopment on this narrow spit between the Mediterranean and the Mar Menor lagoon. The roads onto La Manga are horribly busy.
The Murcian Mediterranean coast west of Cartagena is much quieter and much less spoiled - depends what you are looking for - Aguilas is very small but I think Mazarrón has yacht charters.
Whenever you travel, the flights will be full of golf obsessives holidaying at the La Manga club (some distance to the south of Mar Menor) or one of the other new golf courses springing up between the fields of plasticultura (intensive growing of vegetables in polytunnels) that have replaced too many of the orange and lemon groves on the area immediately inland. You can get some good deals on full service BA flights on days when there aren't large parties of golfers travelling.
There is lousy public transport from Murcia airport, and taxis are pricey, so it is almost certainly worth arranging car hire even if you are going to be based close to the Mar Menor. You can then explore other parts of Murcia province - Murcia city (make sure your other half is a competent navigator to find parking!) , the old towns of Lorca, Caravaca de la Cruz (in spectacular mountain scenery), and the port of Cartagena (probably only a must see if you are interested in Roman remains, although you can get fantastic seafood meals at rock bottom prices) .
Murcia Turistica - official regional tourism website
I can concur with Lang Rabbie on La Manga strip, reminiscent of Blackpool with sunshine but sans the donkeys. We rented a house on the Mosa Trajectum golf complex, although neither of us play. Paid a visit to Lo Pagan one evening and a enjoyed an excellent dinner in a restaurant in a sidestreet of this little coastal town, made all the more enjoyable by it's complete lack of Brits. Murcia city we found to be a complete knockout, the old cathedral, the walkways and alleys of the old town where you will find a host of restaurants and friendly bars at every turn, plus a slew of fashionable shops for both sexes. Not one Rovers Return, English breakfast, or tea as mum makes it establishment in the whole place, and long may it stay that way!