I had troubles with that as well... the only difference I can see (other than some minor differences in appearance) is that the "Caymen" has more size options... I went to a local store to find some
Crocs for my upcoming trip - and they did not have the color I was looking for. When I tried on the "Beach", the only ones this store sold, I noticed that the sizes were like: 6-7, 8-9, 10-11, etc... Well the problem I found was that I was between a 6-7 and an 8-9. So I was happy to discover that the "Caymen" was sold in an 8. I took a chance and ordered them, and they fit perfectly... I don't know why they did that but I can't really tell any other difference.
At first I didn't see that they were very comfortable... but after breaking them in I think they are conforming to my feet. They do have great support, they're light and great shock resistance... The only problem I could see anyone having with them is if they didn't fit quite right.
As for falling off, mine fit pretty well, I can't see how they would just fall off like a flip-flop would... hope this helps...
--Joe
P.S. Check out the flip-flops too, I bought a pair and they are BY FAR the most comfortable flip-flops I've owned... and I've owned a LOT of flip-flops.