On a beach you won't face any issues regarding lack of ambient light until evening. For this a crop body like the 450D or 7D will do nicely.
Using a wide angle is a great idea (especially at empty beaches), but in case you want to shoot some water sports activities you may also want to use a long focal length. Your 70-200 with or without teleconverter will do nicely in this regard, while your 5D Mark III will do nicely for those wide angle shots and for high ISO shots in the evening.
Being me I would leave nothing in the hotel room, even if I happen to be in one of those countries, where theft is normally a non-issue. With your gear I would use this setup:
450D + 70-200mm + teleconverter
5D Mark III + 24mm
Avoid swapping lenses, keep your gear out of the sand and you should be fine.