Matthew Maran / Great created grebe (Podiceps cristatus) with chick following, and person swimming in pond, Hampstead Heath, London, July 2014.

Karine Aigner / View from Block Island looking out to sea, with swimmers' belongings on the beach, Rhode Island, USA. October 2008.

Doug Perrine / Pod of Hawaiian / Gray's Spinner Dolphins (Stenella longirostris longirostris), one dolphin is playing with a leaf draped across its tail fluke, Ho'Okena, South Kona, Hawaii, USA, Central Pacific Ocea...

Matthew Maran / Swimmer about to dive into the cold water of the Men's Pond, Hampstead Heath, London, England, UK. January 2016.

Pascal Kobeh / Swimmers / snorkelers with a Whale shark (Rhincodon typus) swimming over coral reef, Maldives. Indian Ocean. March 2012.

Pascal Kobeh / Bottlenose dolphins (Tursiops truncatus) swimming in open water with two snorkellers around them, Reunion Island. Indian Ocean. October 2009.

Pascal Kobeh / Bottlenose dolphin (Tursiops truncatus) at the surface with a swimmer, Reunion Island. Indian Ocean. October 2009.

Olga Kamenskaya / Swimmers taking a dip in Lake Baikal, mountains visible beyond. Zabaikalsky National Park, Siberia, Russia, September 2013.

RHONDA KLEVANSKY / Four swimmers walking up beach, North Beach, Durban, KwaZulu-Natal, South Africa, August 2009.

Ross Hoddinott / 2020VISION / Sika deer (Cervus nippon) amongst flowering heather, Arne RSPB reserve, Dorset, England, UK, August. Did you know? Sika deer are surprisingly good swimmers and can swim for up to 12km.

Michele Westmorland / Swimmers beneath Annandale Falls. Grenada, West-Indies, Caribbean, May 2009.

Alex Mustard / 2020VISION / Triathalon swimmers, benefit from the good water quality of Southampton Water, The Solent, Hampshire, UK, July 2009