I have never done underwater photography and if I did, I wouldn't want to risk an expensive Leica. The X-U costs almost as much as the Q, and still lacks a built-in viewfinder. I would buy a used X113 or perhaps a Sony RX100 series of Leica D-Lux, but the X-U isn't really on my radar at that price point and still APS-C and no EVF.
A pro photographer friend of mine keeps promising to take my X-U underwater. He shoots mostly models under water, but occasionally does nature too. Hopefully he'll get to this soon.