I believe that the best bang for the buck and the less complicated camera is still the Canon G16 or S120 as she does reasonable small macro to moderate wide angle without any additional lenses.
She has a broad choice of housings and acessories like macro and wide angle lenses and work perfectly with INON strobes using INON's s-TTL system.
A option to create a super small rig would be the S120 8same sensor and processor, but slightly different lens) with 2 INON of the tiny S2000 strobes.
If you "need" 4K video then the choice become less broad:
Keep in mind that photo cameras are quite different to use as video cameras, especially the autofocus don't work well under water on photo cameras.
Could you set the focus to lock position where everything is in focus on the small camera's when taking video, so it won't keep trying to focus on different subjects? I have never used a small camera I probably will be using it as a video camera 90% of the time.
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