The SeaFrogs housing does not have an optical bulkhead by default, but UWTechnics converter comes with a replacement optical bulkhead that threads in place of the SeaFrogs one and has a built-in LED - https://uwtechnics.com/index.php/online-store/ttl-converters/for-sony/16034-ttl-converter-for-sony-a7iii-a7ii-for-seafrogs-meikon-housings-detail
Inon S-2000 is far too weak to be useful with this camera; you want Z-330 at a minimum. Also, keep in mind that S-2000, besides being weak, only has optical triggering, so in order to use them, you need the $460 converter regardless. Retra Prime/Pro are also optical only, so to use them, you need either the UWTechnics converter, or Retra's own e-opto converter - the latter is less expensive, but manual only, whereas UWTechnics supports TTL and HSS.
Your best price/performance option is probably a pair of Z-330s or YS-D2Js and a dual electric sync cord. This will limit you to manual operation of the strobes, and you will have to take care of a number of extra o-rings, but it will give you good power at a reasonable cost. Next step up would be a pair of Retra Pros with a UW-Technics converter - this will give you more power, TTL and HSS capability, plus all the optional Retra accessories, but at a significant step up in cost.
Note that if you decide to get into snoot photography, strobes with an offset modeling light are notoriously difficult to use with a snoot, as the spot of light from the modeling light is not cast into the same spot where the actual strobe light will land, and the actual offset between the two varies with the distance between strobe and subject. Both Z-330 and YS-D2 have modeling lights that are offset to the side and top respectively, whereas the Retra flashes have it mounted centrally, inside the circular flash bulb, which makes aiming the snoot relatively trivial. Some snoots have targeting lasers, but I've seen posts from frustrated users claiming that these are not particularly useful. As far as Retra goes, I was able to produce usable images with Retra Pro + LSD on my second dive with one.
For what it's worth, I'm using an A6300 in a SeaFrogs housing with UWTechnics converter and a pair of Retra Pros.