Dark strip on images is shutter 2-nd curtain. It became visible in shutter window at the moment of flash, because of inconsistent flash sync speed setting in your camera menu.
Nikon D7100 has mechanical shutter, which makes fully open window at max fast speed 1/250. If you set faster speed (1/320 or faster, or FP speeds), then camera mechanism does not make fully open window at all, but just an open strip which moves along the window (2-nd curtain goes after 1-st curtain faster), the more fast speeds cause more narrow open strip.
Regarding "FP" flash sync speeds in camera menu *
Nikon FP is fast shutter speed shooting method, when camera initiates Nikon land strobe to burst 40khz multi-pulses.
But underwater strobe cannot burst 40 khz FP light, underwater strobe burns only continuously in a single pulse. That is why underwater strobe must burn only at the moment when camera shutter window is fully open, to light the full shot. Such shutter speed, when window is fully open, was named as "X-sync speed" in photography technics.
The maximum x-sync speed which you can set in D7100 menu is 1/250: Bracketing/Flash--->Flash sync speed--->1/250. But, without "FP" !! Only this case underwater strobe will light the full shot.
By the way, If you would use any old DSLR camera, like Nikon D50 for example, (it has electronic-mechanical shutter with x-sync speed up to 1/500), then underwater strobe can work even at 1/500. I tested my strobes with Nikon D50 and #11031 UWTechnics TTL-converter, - all strobes worked perfect in TTL at 1/500 s shutter speed.
Regarding Sea&Sea YS-D1**
The 1/200 restriction in User's Manual, - it is a mistake. This strobe works with all available x-sync speeds (without FP of course), the same as other uw strobes. Believe me, YS-D1 works at 1/250 (without FP), i have just tested it again now. You can set 1/250 flash sync speed in D7100 menu and use YS-D1.
I can only assume that Sea&Sea put 1/200 restriction into User's Manual to avoid any consultations about flash sync speeds, because 1/200 works as x-sync speed for all modern cameras.
Edited by Pavel Kolpakov, 09 March 2018 - 05:07 AM.