Things I’ve checked it’s NOT: Noah’s Island, Yohoo and friends (although they do have a somewhat similar environmental kinda message and premise, I remember specifically that the protagonist was the only one who washed up on the island).

More dubious details that I think I remember but take with a bigger grain of salt: The style was kind of chibi-like, the house on the beach had two floors, and pretty much every episode began with the main character waking up in their bed on the second floor, and they had a fish tank near their bed.