In real life, Jason Statham is extremely comfortable in the water. He was a competitive diver, competing in Olympic trials in 1985 and representing England in the 1990 Commonwealth Games.
In the book, the megalodons are pure white, almost luminescent, from living in an environment with virtually no light. This coloring proved too difficult to render in CGI while still looking realistic, so the megalodons were given the same coloring as great white sharks - grey backs with pale bellies - even though this coloration would only make sense in an environment with light.
How and why there are megalodons (believed to have been a shallow-water predator) in a deep sea trench is never explained in the film, while the novels go in-depth with their evolution into an abyssal species (they migrated to the heated hydrothermal vents to escape the ice ages, over millennia gaining a slower metabolism and losing skin pigmentation in the process).
For the giant squid (Architeuthis dux) sounds, the sound editors parked a car in a quiet location at Warner Brothers Studios and put microphones inside the car (and stereo mics and some contact mics attached to the windshield), then went outside the car with two or three different types of plungers and started plunging the windshield. Sometimes, they used a dry plunger and sometimes a wet plunger with dish soap on it to make it slippery and slurpie. The sound editors even came up with some really cool material for the cups of this giant squid. So, they would do a hard plunge onto the glass and then pull it off.
The young boy on Sanya Beach who begs his mother to be allowed to go swimming is another homage to Les dents de la mer (1975). His mother is wearing a similar swimsuit to the one Alex Kintner's (the second victim) mother wore in the original film.