If you think marine biologists can't make money you are sorely mistaken. The problem is that a lot of people who study this field only get a bachelor's and go to work at a pet store, in an aquarium, as a park ranger, etc...
If you go for a Ph.D you make good money. For instance, my fish biology professor makes $129,000 per year without including benefits. Fisheries and aquaculture are very high-paying fields. Here's an article:
http://www.capecodonline.com/apps/pbcs.dll/article?AID=/20081007/NEWS/810070316&emailAFriend=1
I will be graduating with my bachelor's in marine biology at the end of this year. I will be taking a year off before grad school, and I already have 2 job offers in the $30,000 - $50,000 range for that time.
But nobody does it for the money. You really have to be passionate about it. If you're good at it then it helps your chances!