Which branches of Marine Biology are easiest/hardest to get into?

[Eric]

My guess is that the hardest one to get into is Marine Mammalogy, since just the idea of making a living off of researching Cetaceans, Pinnipeds, and Sirenians sounds like an awesome idea to anyone (myself included - it is my dream job that I hope to get one day).

My guess is that the easiest one is probably the study of Plankton, Algae, and small fish (probably less than 18 inches at most), since most websites I've seen say the majority of marine biologists study those three animal groups.

Are these correct, or are there any branches that are harder or easier to get into?