Barmaglot

In addition to autofocus improvements, there's also battery life. Early A7 cameras used NP-FW50 batteries; starting from A7 III they switched to NP-FZ100, which took their battery life from anemic to excellent. If the budget is limited, are you dead-set on full-frame? A6700 is reported to have excellent performance.

I've done blackwater diving with The Smiling Seahorse liveaboard in Andaman Sea; they do at least one blackwater dive per trip, and occasionally they do a blackwater-focused trip (I'm going on one in nine days! Hurray!). The way they do it is that they have a large buoy with a line attached to it, and a light shining into the buoy - this makes the whole buoy glow, so it is visible from a ways away on the surface. On the line, there are pairs of Archon 5400 lumen video lights, one pair at three meters pointing up, another pair at five meters pointing down, another pair at ten meters, another pair at fifteen meters, and finally some lead weights to stabilize the whole thing. All the lights are turned on as the divers are getting suited up, and the line is chucked into the water - preferably, this is done in as deep a place as feasible. The boat follows the buoy, the divers jump in, swim over to the buoy and descend near the line. Nobody is tethered - the line is visible from a fairly long way away, and even if you get too far away to see the video lights, you can find your way back to it by homing on the photographers' flashes. During the dive, the boat follows the buoy as it drifts with the current and picks up the divers as they come up.

I think Weefine WFS07 ticks most of these boxes. 700g land weight, powered by 2x18650, circular tube, 5500K color temperature, 100M depth rating, costs $600. Power is a little on the lower end of the scale at 60W/s, but you can't have everything.

Just checking in, coming over from Wetpixel.