Carp absolutely love sitting in weed. It provides them with a safe spot in which to rest, and because it is photosynthesising in the sunshine, they can enjoy the benefit of resting in a stream of tiny oxygen bubbles while they relax in the sun.
Additionally, weed beds are often packed full of natural food like snails and shrimps, which the carp can graze on in relative safety. Snail numbers can skyrocket in the middle of summer and carp will often spend many hours feasting on these crunchy snacks.
In terms of catching them while they are in the weed, an excellent approach is to use floater fishing tactics, which can enable you to pin-point the larger fish in many situations.
Alternatively, position rigs on the bottom close to the weed beds on shallow areas or in holes in the weed. It is really important, however, that if you are fishing in or near to thick weed beds that you use heavy tackle that will enable you to land the fish safely. I would use a line of at least 15lb breaking strain and a size 4 hook. This ensures that I have every chance of landing a hooked fish.