I am going to put my head above the parapet as a Sea & Sea YS D3 user - and say I'm happy, both with the units and the service I got from S&S. That said, I'm only in the water 3-5 weeks a year, and I am very particular about keeping my gear in order. Sort the gear then the beer, is my mantra!
I shoot M 4/3, and find that the dome diffusers with the 8mm fisheye gives a nice spread of light across the scene. The flat diffusers that come with them I use for every other lens.
I also have the snoot - cheap for what it is, and the prism in it ensures the focus light on the strobe can be used - which is harder to do if one makes one's own. I could see myself getting very frustrated without the light to help know where it's pointing. The out the box snoot is good for WA snooting; for macro it's a bit wide, but there are some easy mods that slot into the snoot and can give a really nice focussed beam. I made my own, and was also given a pair of printed ones to test - am planning to post my findings up one day, with some pics.
I know others have had issues with performance, and with customer service. I posted about an issue I had with my strobe on another forum. That issue had a happy outcome, in which my unit was replaced free even though it was outside of warranty.