While Sony lenses are somewhat expensive, D750 will require a significantly more expensive housing, so it kind of evens out. DSLR housings also tend to be much more bulky than MILC. Compare Nauticam NA-A6500 vs NA-D750.
A6500 has been on the market for a while now, so used options aren't hard to come buy. Same for A6300, which is almost identical - differences are in-body image stabilization (useful with legacy non-stabilized lenses, but you're unlikely to be using these underwater), touchscreen (inaccessible in a housing) and an extra custom button. A6700 is rumored to come out soon, which should depress used A6300/A6500 prices further.
If you're looking at budget options rather than premium housings, Meikon makes a very good and affordable housing for A6xxx series - interchangeable ports, vacuum pump, etc. On the other hand, Sony has well documented issues with white balance (if you're shooting video) and battery life is, at best, adequate.