I would suggest going to a shop that specializes in underwater photography. Backscatter has a shop in New York and they are quite good. You will be able to see and handle the options you might be interested in.
At $2500, you are going to be constrained. It can be done but a bit more does open up options.
I looked at the Backscatter site and you can get a mirrorless 4/3 camera for that price and 20 mega pixels (which should be fine).
Now as to your photo quality, what are you using it for? Off the top of my head, I forget the megapixels in video media. Most of it 4 megapixels max. I think really high end high resolution screens can hit 8 megapixels but I could be wrong.
So the ONLY reason you will need megapixels is for large prints, If you go on you tube, most of the sources say that 24 mega pixels will allow you to print enormous enlargements. For larger megapixel cameras, you get the ability to crop, You can take a photo and compose it in post processing.
By the way, since you are shooting macro, you will probably go for high F values to get depth of field. High F stops result in diffraction which reduces your effective megapixels. So shooting macro means a high megapixel camera is not going to realize its maximum resolution, For you that is good because you do not have to pine for the costly 60 megapixel beasts.
For underwater photography, you want A) a camera that does RAW. B) You have to have a strobe (2 is better but one can do fine. C) the housing - and that is the item that costs $$$$. Also, having a focus light is good too (more than good,
Even a modest camera in the right hands can give impressive results. i used to shoot a Canon G10. There was a guy on the Canon board by the nom de plume of “Gilligan”, who shot a G8 and mostly macro shots. He took some terrific photos. So it can be done. By the way, I think any camera you are even thinking about will be far more advanced than the old G8.
My problem with my G10 was its focus speed was slow so small fish were almost impossible. I upgraded to a DSLR. I went down to Reef Photo and Video. I was thinking of going lower end on the housing. I ended up with the Nauticam. They make great housings with great control and ergonomics. It cost more than I was wanting to pay but the housing was great and virtually bullet proof.
I have a friend who shoots with a mirrorless 4/3 and it does not constrain his shots. I shoot a Nikon z8 with a nauticam housing. It takes great photos. But it costs the GNP of a small country. Underwater it handles nicely but it is big and unwieldy for macro shots. My friend can one hand his camera rig and get into tight areas, Also, he can go from macro to wide angle on the fly. I have to make a decision before the dive to shoot macro, mid range zoom or wide angle zoom. So if I am on wide angle and see a sea horse, well no shot for me.
By the way, try to get a housing that has a vacuum lock system. With a vacuum lock, your chances a flooding your camera go down to very close to zero. It costs a bit but it is well worth the piece of mind.
Good luck to you.