Five things to know about underwater photpgraphy: 

1. You should know how to dive well (duh)

2. For best photos, you have to get close to the animals, with care and quiet

3. Shoot upwards, so that you get more image in the background

4. Learn to use artificial lighting efficiently, as you will be shooting underwater

5. Be patinet (the animals are not paid models).Lastly, do post production using  Adobe Lightroom/Photoshop / other image manipulation software.

The best underwater camera for beginenrs: GoPro Hero4 Silver Edition (affordable, can shoot upto 131 feet, has an extreme wide angle lens which is helpful in closing the gap between you and the animal, Protune for Video + Photos for neutral images)

The best housing for GoPro: GoPro Dive Housing / GoPro Replacement Housing (makes the GoPro Hero4 completely waterproof up to 197 feet or 60 m.)

The best filter for GoPro: FLIP4 One Filter Kit with DIVE Filter for GoPro3, 3+, 4 (helps bring color and contrast to your underwater photos) 

The best video lights for GoPro underwater: Light & Motion Sola 2500 (has a wide beam to cover more area) . Next best is Sola 1200, but quality not as great as Sola 2500.

The best underwater camera housings: Nauticam NA-RX100III housing (made from aluminum, not plastic or Ikelite). Olympus also makes great housings for cameras. Nauticam makes the best camera housings overall.

The best underwater point-and-shoot camera: Sony RX100 II / RX100 III (20+ megapixels of resolution, full manual control, a large sensor with excellent low-light sensitivity, easy sync with an external underwater flash)

The best mirrorless camera for shooting underwater: Olympus E-M1 (mproved strobe sync speed of 1/320 second, which is nneded for freezing action, 10 frames per second with 41 frame raw buffer,  2×2 switch, which allows you to quickly toggle the operation of the main dials between f-stop & shutter speed and white balance & ISO, all controlled with buttons)

Second best mirrorless camera for shooting underwater: Sony A6000

The best strobe lighting (hugely more powerful than a continuous beam light): Sea & Sea YS-D1. Sea & Sea YS-01 Underwater DS-TTL Strobe is an affordable option.

The best spot / flood lighting: Sola 2100 Spot/Flood (small but powerful)

The best video filters (to get good color while shooting underwater): Auto-Magic Filters for Digital Photography Underwater - 50x50mm Square

The best way to check your shooting rig is airtight: Nauticam Vacuum Check and Leak Detection System: M14 Vacuum Valve

Also get some spare parts, batteries, and tools: Keep at least one spare set of batteries (camera & strobe/video light), as wella s memory cards in the dive boat.