Tides
Everyday on a shore the tide goes in and out twice. Click here (BIG file, > 8 Mb)) to see a time lapse of this happening at Bembridge, Isle of Wight, UK.
There are smaller files here (real video, 350 kb), here (Windows Media Player, 1.5 Mb) or here (WMP, 237 kb).
This daily cycle means that organisms living on the shore are alternately in and out of the water (if they are at mid-shore) or very rarely in water (on the upper shore) or very rarely out of water if they are on the lower shore. These facts are behind much of the distribution of animals and algae that we see.
Further explanations can be found here, here and many further links here