One thing that is very useful to know about are waves - how they are formed; the different types of waves and what they do. One of the better explanations for this can be found at http://www.geography-site.co.uk/pages/physical/coastal/waves.html
Another site covering waves (with diagrams that I use in lessons) is http://www.geography.learnontheinternet.co.uk/topics/waves.html
I'll wave goodbye to this post now.... (groan!!!)
