Weather in Amelia Island in April. The average maximum daytime temperature in Amelia Island in April is a warm 26°C (79°F) with low heat & humidity. There are usually 10 hours of bright sunshine each day, which represents 73% of the 13 hours of daylight. The average sea temperature in and around Amelia Island in April is 20°C (68°F). Expect 64mm (2.5 inches) of rainfall across the whole of April with 6 days with some rain falling. Maximum UV levels will be very high (9 UV index) at midday when the skies are clear.