As someone who is currently a biologist (and who works in a lab on the same hall as several marine biologists) most of the work is pretty monotonous - i.e. data collection, data analysis, etc... Granted, I'm only a lab tech but I've worked for a fairly well-established ecologist for a while and I am around marine biologists, ecologists, geneticists, microbiologists, etc... all day every day so I have some idea of what it is like. In our lab, we spend two months out of the year doing the fun work (out in the mountains, hiking to field sites, working with specimens in the field) - the stuff that most people think of doing when they think "I'd love to be a biologist". The other 10 months are mostly spent in a lab extracting, processing and analyzing the data. The interesting parts are the field work, discussing the science relevant to the research you are doing with your colleagues, and the payoff of getting results and figuring out what they mean. However, that is all a very small part of the job.

If you actually have your PhD and are a post-doc or PI, then you don't have to deal with a lot of the monotony that your lab techs do. That being said, I hope you like writing grant proposals, dealing with a LOT of annoying logistics, reading loads of journal articles (some of which are fascinating, many of which are incredibly dull) to keep up new research, working ridiculous hours (I've known people in this field who work 12-19 hour days, 7 days a week for months at a time) and, if you are a post-doc, being underpaid.

All that being said I went out of my way to be pessimistic because I think a lot of people have rose-tinted glasses when it comes to their idea of what being a biologist, or marine biologist, is like. I enjoy what I do, and for the people who are in it because they absolutely love biology I have no doubt that it is their dream job, but you have a pretty intense passion for it to make it work if you want to make more than 25-30k a year doing it.