Have a look at some of the prize-winning shots from the Underwater Photographer of the Year 2017
This picture by Gabriel Barathieu is called "Dancing Octopus". But the eight-legged creature isn't actually dancing, its hunting in a shallow lagoon in Mayotte in the Indian Ocean. Did you know that all octopuses have poisonous venom? But only the small blue-ringed octopuses are known to be deadly to humans.
Nicholai Georgiou/UPY 2017
Check out these amazing orcas! Imagine getting close enough to take a photo like this? Nicholai Georgiou took it while he was taken during an week diving with wild killer whales in Norway. It won the Most Promising Photographer Award. The water was absolutely freezing, so we bet Nicholai was glad to get orca there...
This photo - Oceanic in the Sky by Horacio Martinez - won the Up-and-Coming Underwater Photographer of the Year Prize. The photo was taken in the Red Sea. The shark in the picture is a called an Oceanic White Tip shark - because of the white tips on its fins.
This amazing picture of a diver caught in underwater light is by Nick Blake and it won the prize for Best British Underwater Photograph. It was shot on the Kukulkan Cenote in Mexico. Did you know Kukulkan is named after a Maya snake god?
These green turtles were snapped swimming in the sea near the Spanish island of Tenerife by Greg Lecours. He said it was a magical moment: "It was early morning and the sunbeams pierced the surface... after a little while, the turtles were circling around us". Sounds turtle-y amazing! Some green turtles live till they're 80 years old in the wild.
Every summer hundreds of humpback whales gather off the coast of Cape Town in South Africa for a massive sea picnic. That's where Jean Tresfon snapped this amazing pic. Adult humpbacks grow to 16m long and weigh about 36 tonnes - that's like three double-decker buses!
One of the categories was for sea creatures found in British waters. This is a long-clawed squat lobster in the sea near the Sound of Mull on Scotland's west coast. The picture by Trevor Rees is called The Eyes Have It - we wouldn't want a staring contest with this guy!