In Padstow harbour seagulls have been walking on the ice
Temperatures in Devon and Cornwall could drop as low as -9C overnight, according to the Met Office.
Temperatures dropped so low that Padstow harbour froze over, with seagulls walking across it.
In Plymouth, a homeless shelter said it was providing extra "sofa" space to accommodate people sleeping on the streets and handing out extra blankets.
Animals at Paignton zoo in Devon have been given extra bedding and tropical trees have been wrapped up.
Temperatures in the region are forecast to rise to only -5C during the day on Wednesday.
Rob Atkinson, Padstow harbourmaster, said: "We estimate the ice is about quarter-of-an-inch thick.
"It's thick enough to support a seagull but I wouldn't advise anyone to try ice-skating on it."
Andy Lang, assistant manager of the Shekinah homeless mission in Plymouth, said: "A lot of people have been coming in feeling the effects of the cold.
"We've been trying to provide extra sofa space in our night shelter to supplement the number of beds and giving out blankets to people who have homes but can't afford to heat them."
He said it was difficult to calculate the number of homeless people in Plymouth at the moment but said the mission provided breakfast for 15 people earlier.
Animals, including tapirs, have been given extra bedding
Martyn Rogers, director of Age Concern Exeter, said: "Winter deaths far exceed deaths at other times of the year and elderly people may feel reluctant about going out at the moment.
"As a good neighbour it would be great to knock on doors and check all is well with people."
At Paignton zoo some of the birds have been moved indoors and tropical plants have been wrapped up as protection against the frost.
Spokesman Phil Knowling said: "A lot of species are very adaptable. You get cold nights even on the plains of Africa.
"The most important thing is to stop ice forming on water troughs and in the lakes."
Devon County Council said it had spent £469,000 gritting roads in the county this winter, compared to £188,000 last winter because temperatures have been so low.