By Wil Longbottom
For the unaware snorkeller, coming face to face with one of these enormous underwater mammals might cause a bit of panic.
But these rotund creatures want nothing more than their bellies scratched as they float around the warmer waters near Crystal River, Florida.
Several freshwater springs attract the gentle manatees when their usual ocean habitat becomes too cold in the winter.
Left a bit: A snorkeller tickles a young manatee near Crystal River, Florida, where the mammals gather in the winter to escape colder water
Underwater photographer Alex Mustard travelled to the west coast of Florida to capture the animals as they congregate together to escape the cold.
He said: 'Manatees live life at a slow pace, so when you swim with them you are forced to chill down to their speed.
'They're just floating about in a sort of zen state. Then, when they come up to you and want you to scratch them, it is so exciting - you get a real connection.'
The 36-year-old said photographing manatees was an easy task because of their laid-back nature.
He said: 'Manatees are big and, if you don't stress them, very friendly, so they are easy to get close to the camera.
'If you give one a good scratch, it will remember and 30 minutes later you will feel a nudge in your side - and there is the same individual wanting another tickle!
'The babies are the most playful, and I think the adults are happy to have snorkellers entertain their energetic youngsters while they get some sleep.'
Life in the slow lane: Scores of the hulking animals rest on the river bed
Can I help you? This manatee appears to strike a pose as one snorkeller attempts to capture the perfect picture
Mr Mustard began his passion for underwater photography as a child and every year he travels across the globe exploring everything from shipwrecks to wildlife.
But it is the waters around Britain that excite him the most.
He said: 'Our own seas and rivers are a hard place to photograph because they are cold and usually murky, so for years I ignored them, preferring to travel to find conditions more conducive to producing pleasing images.
'But for the last 18 months I have been working on a British conservation photography project called 2020VISION, where a team of 20 photographers are documenting the importance of the wild places of Britain for both wildlife and the British people.
'Being encouraged into British seas I have photographed everything from eight metre long basking sharks, to mating cuttlefish and secretive seahorses.
'It's made me appreciate our waters more than ever.'
'Sea cows': This adult manatee yawns as he shelters underneath a floating pontoon