Guernsey Alzheimer's Association has become the main beneficiary of the 44th Skipton Swimarathon.
After submitting an application earlier this year, the charity will receive £25,000 in October.
The fundraising event is due to take place between the 7 and 11 October. More than 2,500 islanders will swim to raise money.
Guernsey Alzheimer's Association runs a sitting service. This gives respite to carers looking after people with dementia.
For a few hours every week, the sitter switches places with the carer so they can have some time to themselves.
The service currently has 16 sitters but nine more have recently signed up.
In Guernsey, more than 1,200 people have dementia. Charity manager, Julie Thompson, says "demand is constantly growing for the service".
The increased funding means the charity can double the service over the next year.
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