You don't need a full frame body and Macro lens for taking photos of fish and coral.
Unless you're planning on making a business of it.
The magician and not the wand and all that ...
An iphone and a clip on CC filter is all you really need ...
I thought I'd clarify why I said the above with some reasons.
I've worked as a photographer for over 40 years now, so I've shot pretty much everything.
Once you start taking macro lenses you are talking about lens which because of their design have a very shallow depth of field with objects located close to the lens. In order to expand the depth of field you need to stop down the lens a lot, often to f32 or smaller.
Doing that means that you will either need to use a long exposure time, or you will need to increase the ISO setting which increases noise in the image.
Increasing exposure time works with static objects, but not so well with moving fish or coral. The increased noise can often these days be reduced with noise reduction algorithms.
If you start using a macro lens with a larger sensor (either Full Frame or Medium Format), the problems with depth of field get even worse.
Bearing in mind that you are going to have to work hard to get a decent image, then use NR to clean it up, then probably need to edit in Capture One, Photolab or Lightroom - you're getting to a point where you have to ask if it is worth all that cost and effort for a low resolution image that is going to be displayed at 72dpi on a web page or in socials.
Of course if you're planning on a business doing commercial photography like on the ocean reefs, then it's worth the money because you will get well paid for it. A friend of mine dives reefs all the time for commercial work and loves it.
You could be keen shutterbug, in which case I'm sure that you'll not just be buying gear to take coral photos, so again it may be worth the cost and effort.
But, for someone who just wants to post photos of fish and coral on forums and socials, pretty much the best bang for the buck is a decent P&S or phone camera with a set of color correction filters and perhaps a clip on macro lens.