The general sentiment that is pro-TTL for macro, and anti-TTL for WA, is not to do with subjects, but with what works.
In general, the entire frame is filled in macro, and the subject is a good part of it, so TTL systems tend to work well.
In general, the entire frame is less filled in WA shots, with lots of open water. So TTL systems tend to have a harder time getting the exposure right. Though I find this far less true than it used to be.
Many macro subjects too, can swim away and hide quickly, like long nose hawkfish, or tiny baby wrasses, so you may only get one shot at them.
I wish there were a TTL system that would just apply the F number guide rule, based on focal distance as measured by the lens, apply the right amount of light accordingly.
Yes, there are lots of uw true TTL systems. They are mostly for DSLRS by canon & nikon. Ikelite makes housings and strobes that do proper eTTL or iTTL with their respective cameras. A few canon P&S digicams also work with TTL.
I gather sea and sea makes TTL solutions that work with canon & nikon, in their housings as well.
Housing a land strobe is not often good, because the color temp is wrong, and the power is not there. But since there are lots of TTL solutions that work fine, it is not a concern.
Many or most folks shoot sometimes/often/always with diffusers over the strobes. Not a problem with TTL. Not that much a problem with manual - just adjust by the right number of f-stops...