In any investigation when you finish ruling out options the remaining option is the likely cause. IDF has ruled out shelling from the sea, a rocket attack from the air and shelling from the land. Hamas rushed to the site to clear the evidence and the Palestinians have not been forthcoming in helping the investigation. The most likely cause at this point is a Palestinian explosive device.
Yesh Atid MK Shai Piron resigns from Knesset (Haaretz)
from the article: IDF hard-pressed to pinpoint cause of Gaza beach deaths