I work in forestry. I was in natural resource management but now I'm in research and also do consulting on the side.
I have an insight into fish management but not marine biology; you'd take similar courses like ichthyology but focus more on community ecology, population dynamics, etc and have a fairly different view on natural resources than a marine biology program would lead to. That would lead to careers like working for fed and state agencies like fisheries, do population monitoring, etc. Eventually you'd go into program management and spend more time in the office dealing with budgets, logistics, planning as you advance. You'd like start off around 30K and top off around 70-100K (unless you want to work for organizations like the Nature Conservancy in which you'd be paid less but ideally enjoy the job more). So it's fair for middle class living but you'll likely never be a high earner.
I actually wouldn't worry about job prospects. Whenever government jobs decline, the work still has got to get done by public agencies, and developers still have to comply with regulations and laws (NEPA, ESA, etc) so consulting work, which pays alot more, picks up.
So that's fish/fisheries management type of work. Can't offer insight on marine biology.