A comment Tip No8.
If you go to the beach, take a ice cream container with you and fill it up with sand, and other things wich wont look out of place on your models.
If you do this it is a good idea to wash the bits before you use them; or even bleach them. This will aid gluing and stop any nasty reactions between paint/glue and whatever is floating around your coast.
Also, watch out for hermit crabs; my grandmother ended up with one loose in her pocket after beach-combing.