I've dried out, admittedly in our aluminium lift keel sailing yacht, at Wittering beach, Bosham against the quay, just inside Bembridge on the left (very technical description, but it's beside the Bembridge sailing club!), far up Newtown Creek, up the Beaulieu River, Christchurch Harbour, and most commonly given we are Poole-based, Studland beach, Swanage Bay and the south side of Brownsea Island.
With the aluminium hull I don't worry too much about the bottom, but would certainly prefer sand in the GRP-hulled RIB. We always go in on a falling tide, work out how much it has to fall, and depending on how long we want to be dried out, anchor in the appropriate depth of water. As per previous comments, worth checking the area in advance if you can at a lowish tide, so you 1.) know what's on the bottom (sand, stone, rock, mud, etc) and 2.) know how steep it is, is it a flat drying area, etc. if we're staying on the yacht, we don't really want to be leaning over for 6 hours in the night!
And definitely pick a calm area and weather window - the last thing you want to be doing is trying to get off a beach as waves keep lifting you up and depositing you back on the beach. Doesn't do the boat, or you, any good! Preferably anchor where the wind is blowing you off the beach, and/or put out a stern/kedge anchor as well to help you get off, and ensure you don't end up further up the beach as the tide comes back in.
Once you've done it once, you'll probably find you do it again! We certainly regularly do in the sailing yacht all along the South Coast, over in Brittany, and find some wonderful sheltered places to hide in.