We decided to call them hybrid because the have the body and the handiness of a compact with the soul of a reflex. Well, not just the soul, because reflexes have at least two fundamental characteristicsche that rate them above the compacts: a large-size sensor (Micro Four Third or APS-C) and interchangeable optics. Like compacts, they have a wide range of automatic functions, like compacts and prosumer compacts, they largely admit manual control. Compared to reflexes, they lack  pentaprism, mirror and viewfinder, and this allows for a drastic size reduction of body and lenses and, by consequence, weight.

The main technical characteristics for underwater usage are:

Availability of underwater housing: indispensable, of course.

Max operating depth: depends not only on housing resistance, but also on operability of buttons at high pressure.

Strobe diffuser: fixed or mobile, recommended for better performance.

External strobe availability: depends on the presence of a hot-shoe connector on the camera and on the case (either by cable or hot-shoe), or on the possibility of controlling an external strobe using an optical fiber cable connected to the on-camera strobe.  

                                 

Availability of additional lenses: some housings can mount macro and/or wide-angle lenses for improved versatility.

Sensor size: at equal megapixel number, a larger sensor ensures a higher signal-to-noise ratio, and therefore a better image quality.

Macro: the minimum focusing distance in macro mode, expressed in cm, gives a measure of the quality of small abject images; ina addition, the minimum focusing distance in tele mode is also important because the strobe light diffusion is more uniform at not-too-close distances.

Stabilizer: very useful because cameras tend to select long exposure times underwater with ISO not too high (high ISO's are not recommended for good image quality).

Underwater specific modes: dedicated modes optimize the relationship between available light and strobe light and often but introduce chromatic corrections that can worsen image quality.

Display dimensions: in inches: the larger the viewer the easier is subject framing. Visibility in strong light conditions is also important.

Electronic viewfinder: an add-on thet makes it possible to operate in-housing like with a reflex; a critical element is resolution, in pixels/inch.

Megapixels: a higher number of pixels is not necessarily a synonym of better quality, which instead is mainly related to individual pixel dimensions.

HD video: ensures better video quality.

Price: in US dollars "from"; should be judged considering both camera features and comparison with equivalent non-amphibian cameras with separate casing.