Pullbear, this is a seriously complex issue, and one filled with lots of questionable information. The first question is "should I be using a full frame camera for taking super macro images?" The answer is yes and no. You can do it, but you will have all sorts of limitations. You will need a super strong wet diopter. You will have reduced image quality due to diffraction from using a super high F stop, the object will be way too close and difficult to find due to the huge size of the camera. A long time ago, I used a 4x5 for macro images. Worked great in a studio, and really, really sucked to use in the real world.
So what should you use: Well, if you think of:
1. Full frame
2. Cropped sensor
3. 4.3rds (half sized, but actual 1/4 area)
4. Hight end Point and shoot.
Cropped sensor is a far better choice, but in most cases the difference is pretty small. You get better magnification, but only by 1/3...more working space, but only by 1/3 and only 24 meg images (that is a joke, 24meg is equal to a 36 meg full frame. So for super macro (and only supermacro), one can get a slightly better image than a 36 meg full frame can do.
The high end point and shoot has 20 meg sensors...and if lens were equal to that sensor, would be ideal, however there are lots of limitations and for the most part, while they can take good images, there are way too many limitations (limited f stop and focus issues just to name a few).
But the 4/3rd by accident happens to be in the sweet spot. There are several ideal lens....they can do 2/1 at the same distance as a full frame can do 1/1.....because they use smaller macro lenses, depth of files is better.....and they current have far better resolution (20 meg micro 4/3 for super macro is equal to a full frame with an 80 meg sensor).
None of the above was done by design...it just worked out that way, and has nothing to do with taking any other type of image.
So why point this out...well it effects what wet lens you use, and the image quality you get.
I current use the Panasonic GX8 (there is now an Oly with the same sensor), and I use the Aquako iV. I have the subsee +10, and gave away the SMC. I need to buy the Aquako III, as the IV is only usable in some, very calm conditions.
If I had a full frame, I would need a +5.00 to get the same results, but it would be way too close, way to difficult to focus and you would have to use the highest F stop you had.
In truth, that 2/1 covers most things I would take images of, and I can use F8 (the sweet spot for image quality) most of the time.
So if my dive buddy had a full frame, and used this new 3.5... I can take the same image with a 1.75...except I would have twice the working distance, be able to use half the F stop and would be a lot easier (although find and focus would be the same for both cameras) Using less glass, all things equal, my image quality would be better.
If I used a 3.5, that would be 7X magnification, and would be better called a microscope..might be fun to try though.
I so wish all of the above was not true, as I dearly love full frame camera's...and larger.
So, I'm no longer looking a stronger wet lens, I'm looking for the best image quality one around +2