Camera for diving and travelling

Do you need telephoto capability while trekking? If not, something like a Sony RX100 II or Canon G7X (optionally mark II), or even an Olympus TG-5, paired with appropriate housing will cover your needs. Since you're diving to 30m, TG-5 will still need a housing, but it will require much less babying than a normal compact camera on hikes - if you drop it, or get caught in a rain, or fall into a creek, it will be fine, whereas an RX100 immersed in water without a housing or dropped on rocks will not survive the ordeal.

If you do need telephoto capabilities, then you'd be looking at an interchangeable-lens camera, because superzoom lenses require extending outwards to work, which makes them impossible to fit into an underwater housing. A Sony A6000 with 16-50mm and 55-210mm lenses is quite compact and lightweight while covering 24mm to 315mm FF-equivalent focal range. It's listed for $798 new at B&H (including camera, both lenses, spare battery, memory card and a bag), and I see used kits offered under $600 on Ebay. A Meikon housing to take it diving will run you $150-200 for a basic fixed-port model, or $327 for one with interchangeable ports and vacuum sealing capability.

Alternatively, if size and weight are of paramount importance, consider an action camera - either a GoPro, or one of its knockoffs. Image quality will be nothing to write home about, but it'll be tiny an unobtrusive. As far as diving goes, larger cameras tend to dominate your dive - you dive to take photos, and the camera rig requires constant uninterrupted attention and care - whereas with an action camera, you can clip it off to your BCD and forget about it until you need it.