Forget MF, it's way too slow to useful. The only real use case for MF macro is when you're shooting supermacro with a diopter, setting focus to minimum distance and just moving the camera until subject comes into focus, and the subject must be completely stationary.
Keep in mind that with the SeaFrogs housing, you don't have much of a choice when it comes to ports - there's the short macro port that fits Sony 30mm macro and the long macro port that fits Sony 90mm FE macro. The latter is 130.5mm long, and the port fits it quite precisely, so that's your guideline for lens length.
Tokina 100mm extends to focus, so it won't fit in a port. Same goes for the Sigma 70mm DG. Sigma 105mm has internal focusing, but it is 135mm long, so it won't fit in the available port either. Sony 50mm has some... very unfavorable reviews. I don't own one, but by all accounts it's quite slow and clunky, and the image quality is not great. It extends to focus, but it's short enough to fit into the available port even so. Still, I don't know how well, if at all, would it work with add-on wet diopters.
I shoot with a Sony A6300 in the SeaFrogs Salted Line housing, and my first macro lens was the Sony 90mm. It's got great image quality, but focusing is fairly slow, and 90mm focal length on APS-C crop is quite tight. After trying it on a blackwater dive and failing miserably, I went on Ebay and bought a Canon EF-S 60mm f/2.8 Macro USM, a Metabones IV adapter, and a Sony 30mm f/3.5 Macro. I'm not particularly impressed with the latter; the 30mm focal length is just too short and wide for proper macro shooting, it's more of a fish portrait lens, but the Canon 60mm is now my primary macro setup. It's not as tight as the 90mm and focuses much faster, although it does tend to miss focus lock and hunt in poor lighting - having a focus light is extremely important.
I still use the 90mm, but only when I expect to encounter something really really tiny - in those cases, I bring a Weefine WFL-05S close-up lens along, and this gives me a maximum 2.3x magnification, for a frame width of approximately 1cm. Example shot:
Those are clownfish eggs; each one is less than a millimeter in length. Here's a non-magnified shot of the same patch for a little perspective:
But most of the time now, when I'm setting up for macro, the Canon 60mm is my first choice. Its biggest limitation is that even with the adapter, it is somewhat shorter than the Sony 90mm, which causes it to sit recessed behind the port glass, which precludes the use of diopters for extra magnification.
