You have to realize that it is not all swimming around on coral reefs.
That is simply not what marine biologists do 99% of the time.
Most of the time they are competing for grants. Grants they need in order to do their work and live. And there are loads of other marine biologists, nearly all with PhDs doing the same thing, for a very limited amount of money. They are not necessarily researching what they love, they are researching what will give them the next grant, at least until they are very well established. As that grant starts to run low, panic sets in for many. The grant may also fund their students or postdocs as well.
And then there is the writing of papers...paper after paper, because that will impact your grants. It is not just writing the papers, it is getting them reviewed, and jumping through every hurdle to do that, while the people who are reviewing you may well be in competition for some of the same funds.
You go to conferences, give talks...maybe if you are lucky you have a teaching job somewhere. Then you get to worry about tenure, and that also ties back to grants and papers. Maybe if you are lucky you have a field season for a few months of the year, and travel, on your grant money.
It is worth it if you love it, like anything else. You will never be rich, you may very well take a long long time to even get out of debt from graduate school - and you pretty much will need graduate school, at least a Master's. There are marine biologists with PhD's who are unemployed and competing with you.
This helps explain the remarkable number of times on these boards where people, talking about their LFS guy, can say "my LFS guy is a marine biologist and he says..."
But it may still be worth it if you are honest with the reality.
In your doctoral work, which means several more years living on ramen noodles, you get to jump through every hoop that your advisors see fit...along with the grant writing and papers. You get to meet the other students who you will be in competition with, or marvel at the other scientists, long established, who feel you must be kept in your place from the start. I've seen some ugly conference presentations.
You had better love it and you had better be realistic and not expect to have a lot of money or illusions of months spent on a reef. This is the same as the "dolphin" scenario that drives people into marine biology too.
Don't get me wrong, I loved it...but I was a realist as well. I loved it while I had the opportunity to study. Then I had to pay bills.
Marine biology, IMO, has relatively little to do with keeping tanks (many public aquaria people cringe at hiring marine biologists as many have a big attitude - I know, I had to convince them I wasn't like that). Yes, you do need to be able to understand taxonomy, chemistry, even physics to be well rounded as a marine biologist. The more well rounded you are, the better your odds of success.
Note the past tense I've used. I had to pay the bills, and I wanted a decent family life with some nice things....nothing flashy, but nice things. I still owe loads of money for the opportunity to go to grad school. It was fun, I enjoyed parts of it. But it is far more than a nice day at the beach.
I do not generally recommend, BTW, an undergrad major in marine biology. I would do a general biology - with exposure to a lot of different branches of biology. This will both give you a better foundation and also make you potentially more usable as faculty (you may be able to teach more than just marine bio). Don't skimp on math, chem, etc.
So now I work managing people who work in data management. Quite a few taxonomists, actually, end up in data management. I still have my ties to that world...the former fellow grad students desperate for their next hope at a postdoc or interview or grant, month after month. It is not for the faint of heart.
Now some closely related fields, like fisheries biology or aquaculture may be better choices....
As an aside, marine biologists, in my experience, don't always have the best opinion of this hobby in many cases. It might be off base, but it is not necessarily the best opinion. They may collect fish for themselves to study (often dead)...but not for the hobby. Many collectors in this hobby are local people.