Many have already voiced what being a scuba instructor solely is like. I'll chime in on the Marine Bio aspect.
First, your likely career choices with marinebio + scuba in general are as follows:
Marine Field Technician, Aquarist, research and academics, and AAUS Dive Safety Officer
These are Salary positions, some places still pay by-the-hour but you get benefits with a full-time employment.
Marine Field techs can do anything from setting up experiments in the field and maintaining them, so fixing the head on a research vessel. If you manage to land a career as one, you'll likely be doing both of those and everything in between. Often times you're not diving as part of the job, but it happens to be a perk. Most of the jobs are on a ship, so having handy man skills such as plumbing and electrical work is a bonus. Having an education in biology, chemistry, or engineering (robotics) is what will get you the networking and equally important, the qualification for the job.
- For this you do not have to have a BS or BA to get the position, but having it does help out a lot. If you don't have it, you better have a Journeyman in some trade that is relevant.
Aquarist work in aquariums and take care of fish. You need a college level understanding of general chemistry, biology, and other things that are learned in a bio or marine bio undergraduate degree.
Research and academics are the careers working from a university, either researching in the field as a graduate student or PhD postdoc, or teaching academics as a PhD professor. Lots of time spent back at base in the lab (about 90%) Most of it spent writing grants if your at the top of the totem pole. You will want to pursue a graduate Masters of PhD for this career. Expect at least 8 years of your life in school; 4-5 for the undergraduate, and 4+ for the graduate degree (this includes 2 separate university applications w/ recommendation letters and all that jazz).
Dive Safety Officers work as the head entity for various AAUS (American Academy of Underwater Sciences) scientific research communities with diving outfits. They set the standards with their Diving Control Boards and enforce those standards. They also often train the next group of sci divers and in a university setting often they train the recreational department of scuba divers.
To qualify for this position you have to have a professional scuba rating. Assistant DSO's can be Divemasters but DSO's positions usually require an Instructor rating and previous experience in an AAUS or scientific outfit.
It helps to go to a college that has an AAUS Program, join the program, and actively dive with the program to network and gain experience. It's a hell lot of fun too, but also brutal. Imagine not wanting to dive because it's raining, you're cold, but you can still handle conditions. So you dive for the advancement of science anyways. Oh yeah, and there's no shower, you're camping, and the toilet paper box was flooded while you were out.
Your marine bio degree will come in handy for all these positions. You do not have to pursue a degree in marine bio per-say, many people with these positions are just regular biology, ecology, oceanography, environmental sciences, etc etc. Not many colleges actually offer "quote for quote" Marine Biology as a degree, even though they have marine bio classes.
The availability of these careers are very far and few between. People who get these careers often love their job and only leave to either retire or to transfer to their ideal goal which may be working in their choice location or a world renown establishment. Be prepared to volunteer your butt off for experience even after graduating, meet everyone you can, and most importantly: be prepared to move out of state or even out of country for short or hopefully long durations at a time.
Feel free to PM me if you have any questions regarding Marine Bio undergraduate degrees, DSO, or Aquarist careers, as I'm actively working towards those.
Edit:
Also in case none of these careers interest you, having some education in the life that you'll be viewing underwater makes it that much more enjoyable. And generally students will ask their instructors for tons of tid-bits about the animals they see. So it's great to know about stuff and be excited about it.