The number of deaths off the UK coastline reached a five-year high of 168 in 2015, new figures show.
A further 385 lives were saved in "near-fatal" incidents, the Royal National Lifeboat Institution (RNLI) said.
More than half (52%) of those who died had been taking part in activities such as walking, running, climbing or angling.
Coastal walking and running accounted for over a fifth (21%) of last year's deaths.
RNLI coastal safety manager James Millidge said the figures suggest people are "not taking enough care" along the coastline.
"We're warning people to stay away from cliff edges - particularly where there is slippery, unstable or uneven ground - stick to marked paths and keep an eye on the water.
"Watch out for unexpected waves which can catch you out and sweep you into the water," he said.
The charity is entering the third year of its drowning prevention campaign, Respect the Water, which aims to halve accidental coastal deaths by 2024.
It is targeted at men because they account for the majority of fatalities - 84% in 2015.
Mr Millidge described cold water as "a real killer".
He said: "People often don't realise how cold our seas can be. Even in summer months the sea temperature rarely exceeds 12C (54F), which is low enough to trigger cold water shock.
"If you enter the water suddenly at that temperature, you'll start gasping uncontrollably, which can draw water into your lungs and cause drowning. The coldness also numbs you, leaving you helpless, unable to swim or shout for help."
Mr Millidge added: "If you're planning to get into the water be aware that, even if it looks calm on the surface, there can be strong rip currents beneath the surface, which can quickly drag you out to sea.
"The sea is powerful and can catch out even the strongest and most experienced swimmers."
The fatality figures were published by the RNLI following analysis of the National Water Safety Forum's water incident database.
It is the highest number of deaths since this data was first recorded in 2011.