Many of the newer cameras will do 4k video these days, but the problem with frame grabbing is the resolution is relatively low and many frames have faults that are not noticed when the video is streaming. 4K video is around 8.5MP equivalent. And the way you frame the shots is different between video and stills. Video you need a flow and a story to make it work or it's just a bunch of clips strung together, stills you need a balanced composition and attention to what's in the corners and even corner sharpness can be more noticeable in the still frame. You see each individual frame for 1/30 second or so in the video, but you often sit and study a still frame for many seconds and see all the problems with it more readily.
To get the best out of still images strobes really make a difference, though at a pinch powerful lights can get you part way there in specific circumstances.
I think the other question to ask is how experienced you are at UW shooting? - video or stills. Stills you can shoot lots of frames to likely get something you like, but getting beyond a short clip of a fish or manta ray or whatever swimming past I think takes a lot more experience and you are probably more likely to get something you like is you do at least some dedicated stills shooting.
Rather than looking at 4K capability perhaps more important is how the camera captures video - you want it to use all the sensor so you don't lose angle of view if you are doing wide angle work.