I like the idea especially the cooler and guy with red hat. It seems to represent the best light source from the beach. One suggestion for you...perhaps lighten or 'brush' away the tops of the shadows in the layer mask with a 50% opacity brush. This might help give the shadows some depth. Also, if you squash the shadows a bit...more like 'flatten' them by distorting them they will have a more believable plane. Does that make sense?

I agree with pixelkid with the shadows perspective, I would work a bit longer on some of the masking. Mostly the woman on the right around the head region. Also The guy behind the towel keeps distracting me. I don't know if its the location or that he is right behind the towel. I would try to move the guy closer to the ground and see if it helps with the composition. Although I could be wrong. Good Luck!

I have uploaded a better version, (I hope), further suggestions are welcome.

Using Gaussian blur on the shadows will make them less sharp (and warp will shape them also) , and running a blur tool brush over your masking edges help images to blend better (Using distort to make the shadows more perspective correct helps a lot as well (if you don't have warp) )

But the more you practice the more you figure things out, this is a great effort.. (Reduction of the amount of floating things might help as well, less is more ) but that could just be me and always IMHO