For rocky bottoms the grapnel is probably your best bet and a plow anchor would be 2nd choice in my opinion. 

For the Dory a mighty mite may be the trick, since it has bendable tines and is a bit more compact.

http://www.wholesalemarine.com/sea-sense-mighty-mite-hook-release-anchor.html?gclid=CPDZsNDY-c4CFZWDaQod-WwAGA

   I use a 1.5kg folding grappling anchor for my Skerry - mostly rocky bottoms around here - but I don't camp out overnight. A bit heavier one might be better for both the dory and for spending the night on the boat. They fold up nicely and take up little room when not in use. Don't forget a bit of chain!

   One thing I forgot to mention is that when anchoring in rocky bottom with any anchor you should rig them to be able to break-away when trying to retrieve them   Grapnel anchors do a good job, so good that often times they get wedged in the rocks and are almost impossible to retireve unless they have bendable tines, [see mighty-mite anchors] or are rigged to break-away per the drawing below.  I use zip ties instead of nylon line or twine, as I find they hold just as well, are easier to put on securely and given enough strain will break easier then the twine.  I have used this menthod for years not only on grapnel but on Danforth type anchors in ocean and seaway conditions and have never lost an anchor or had it fail when deplolyed.  As for the 2 anchors I did lose, before learning this method, my wife will not let me forget, as much as I would like to LOL!

BTW you can also replace the cable with chain or a good quality rope...not sure how to retrieve when stuck then PM me and I will explain....

Thanks. That's exactly the info I needed -- especially the photo of rigging the anchor to break free when snagged.   

   Interested in your cot idea ,I'm looking for ideas to sleep aboard my dory as well.right now I'm considering removable panels to span the benches ,possibly laced together, with a tarp and bivy tent

   Most every anchor will fail if you do not have enough scope.  Anchor rhode scope is feet of line per feet depth of water.   So if you are anchored in 10 ft of water with 10 ft of line out any anchor will drag.  They need to lay on the bottom and pull horizontally.  Chain on the anchor helps for the first few feet. I suggest you start with a 4/1 scope.