The 34-year-old accountant from Leicestershire was attempting to swim the Channel to raise funds for the two charities.
A statement on the swim’s Facebook page, posted today, said: "While attempting to swim the English Channel yesterday, my sister Susan collapsed suddenly in the water. She was immediately recovered from the water and treated on the support boat. She was then airlifted by helicopter to a hospital in Boulogne. Susan tragically passed away.
"Thank you for your messages of support. Please respect the family’s privacy while they come to terms with what has happened. If you would like to leave a sign of respect, please feel free to donate to her fundraising page."
Barbara Young, chief executive of Diabetes UK, said: "We are extremely sad to hear of the tragic death of Susan Taylor during her Channel swim, which she organised independently to raise funds for us.
"We have spoken to Susan’s family and have offered our support and our heartfelt condolences. Our thoughts are with Susan’s family at this sad time."
Geoff Ellis, chief executive of Rainbows Hospice, said she was part of the Rainbows family: "Susan was a wonderful woman who would do anything for anybody. She has been a much-loved ambassador at Rainbows for more than two years, helping out at events and tirelessly fundraising for us.
"All of the children, young people, families, staff and volunteers at Rainbows are devastated, and our thoughts and prayers go out to her family. She will be sadly missed."