During a visit to relief and emergency service workers in Stainforth, she said Prime Minister Boris Johnson should apply for cash from Brussels to repair the damage.
She praised the work of volunteers, calling it "the best of human spirit" but added that the government is not "taking this as seriously as it should".
"It should be declaring a national emergency so they can open up the ability to apply to the EU for the emergency funds that are available at times of extreme floods," she said.
"They are not yet doing that and they should be doing that."
The prime minister will chair a meeting of the Cobra emergency committee today in response to the flooding.
If her party wins the general election on 12 December, she said it will put money towards a five-year plan to protect vulnerable communities most at-risk from extreme weather.
The flood prevention money would come from a capital infrastructure budget funded by borrowing, Ms Swinson said.
She said: ‘These floods are devastating for local communities and highlight the need to invest in our flood defences.
‘The Liberal Democrats will create a £5 billion flood prevention and adaptation fund over the next five years to support small community and council-led schemes to reduce flooding and protect at-risk communities.
‘Leaving the EU will mean that we lose access to the EU solidarity fund for flood relief and the EU structural investment funds that help maintain and strengthen flood defences and improve flood resilience.”