Underwater photography and videography is a perfect "gadget sport", with many brands, styles and levels of equipment to choose from. It doesn't have to rely on expensive and high maintenance equipment. I started out with a basic compact camera (Canon ixus750) in a Canon plastic case and some of the images on this website were taken with that camera. Its a perfect system for the beginner who may be using photography to improve their diving skills.
Many of the photos on this website were taken with another, albeit more sophisticated, compact camera, the incredible Sony RX100. It is a very popular compact camera for underwater photographers, and many awards have been won by photographers using one of these (including me ;-). This camera has a large sensor for the compact category, can be used in fully manual mode and with external strobes, and is perfect for the competent diver and photographer. Its versatility make it great for switching between macro and wide angle subjects on a single dive, something that can't be done with many bigger cameras.
My primary camera is the acclaimed Panasonic Lumix GH5. As a "mirrorless" micro-four-thirds camera, it offers interchangeable lenses, but being a relatively compact size when compared to classic SLR-cameras it suits the travelling diver. The GH5 is well known for its video capability, and shoots in 4K ultra high definition - the new standard in the video world. The rest of the field is catching up, but Panasonic is still leading in terms of capability and portability.
I am a big fan of NAUTICAM underwater housings and trust in them for both of my cameras. Highly dependable and robust, they are extremely well engineered, ergonomic and still relatively compact for travel.
Lighting is everything in underwater photography, whether by natural light, strobes (flashes) or by LED lights. For flash photography I use INON strobes, which are compact and high performing.
Are you starting to get the theme? Size is important. Not only is it easier to travel with smaller equipment, it is easier to handle in and out of the water, and allows you to get close to subjects with less risk of damaging the reef, or scaring the critters away.
Aerial imagery is via my DJI Inspire 1 Pro. This incredible piece of equipment is bridging the gap between remote control flyers and videographers, and allows me to capture cinema quality footage. The drone is capable of high speed and long range, and captures 4K video with its on-board camera. Key features are its stability and smoothness and is its unrestricted 360 degree view, as the arms lift out of camera view after take-off. This has significant advantages for filming, without interference from spinning blades, landing legs or shadows. None of the distorted and unstable footage that you get from a more basic drone with a go-pro-type camera. This is no toy. With the fully manual X5 (micro 4/3) camera installed, high resolution still photos can be captured using various lenses and filters.