When I searched I found it difficult to find any information on this lens for use underwater before buying it. Having now bought one and tried it, I thought it worth a short post. I dived in the Menai Strait yesterday (very good vis for the Strait at ~5m and 10 degrees C), first time out with an A7II in a Seacam housing with the 50 macro lens - using a Seaflash 150 manually for illumination.
The lens is a 'conventional' (extending) type design which is not as convenient as an internal focus type lens but retains its focal length 'better'. This does mean that a longer port has to be used to allow for its extension during focus. Its focus speed is also quite slow but I bought it for use in UK waters and mostly for benthic stuff so this isn't too much of a worry and in practice it was less problematic than it sounds. I had concerns about battery life with the power used to drive this lens too but found that an hour long, coolish dive didn't flatten it so this was unfounded.
Anyway, results. Despite using the rear screen only and there being slight current at times (we went in just before slack and got out just after - as is the way with the Strait) focus was very accurate indeed, even on small subjects. I didn't push it down to 1:1 but got close. Mid range was excellent too. Here's one shot worth posting of something I've never seen here before. An incredibly slippery butterfish caught by a velvet swimming crab. Its very crisp indeed at f/14. I look forward to shooting with this lens - its surprisingly light in weight but an excellent performer.