Renee Capozzola scooped the top prize with her magical photo “Shark’s Skylight.”
This years judges were former marine biologist Dr Alexander Mustard MBE, photographer Peter Rowlands and author Martin Edge.
A statement from competition organisers said: “The UPY team would like to thank all the talented photographers who supported this year's competition with their pictures, especially in these challenging times.
"We hope that this year’s stunning collection of winning images provides a welcome escape to everyone who enjoys them and a chance to reconnect with the underwater world.”
Winner Renne commented: “I am so excited, truly honoured and honestly, very surprised to be named the Underwater Photographer of the Year for 2021.
"Words cannot describe the level of excitement that I’m feeling for receiving this award, especially this year.
"The fact that a picture was chosen is very rewarding as these animals are at the risk of extinction world wide.”