Other
Beloit College: K 12 Geology
This is a good source of basic information about density currents in water. It includes simple activities you can do to demonstrate these currents.
Other
Lunar and Planetary Institute: Oceans From Space
This collection of photographs from space-shuttle missions shows some features of oceanic circulation that simply cannot be seen from any other vantage point. This site is a great resource for many different topics of study.