The BBC story is here.

They included a comment from Human Rights Watch that is interesting:
An expert working for the Human Rights Watch said the Palestinians' injuries were not consistent with a blast taking place beneath them.

"It has been suggested by some that the family was killed by a land mine, and this is patently not the case," Mark Garlasco said.

"All of the evidence is pointing to a 155mm shell as having killed and injured the Palestinians here on the beach," he said.

"My assessment [is] that it's likely that this was incoming artillery fire that landed on the beach and was fired by the Israelis from the north of Gaza."
If this is a Hamas IO, it's already turning out brilliantly.