It's equivalent to a term or year abroad, only intead of going to university in another country, you go on a research boat. The students take classes and do research. My friend (lived in London until she was 9, then moved to Oklahoma, and had the most lyrical and confusing accent) did this as a sophmore or junior (back in early 1990s) and LOVED it. She was a civil/environmental/water engineer, so it made sense for her degree also, but there were all sorts on the boat.
Make sure you either don't get super motion sick or can work around it before signing up, though.
-Erika