Sarcophagidae?
Sarcophagidae?
I.D. by Stephane Lebrun on Diptera.info (https://diptera.info/forum/viewthread.php?forum_id=5&thread_id=47178)
On seaweed. @sbushes this was a large and really smart-looking fly - I've no idea what it is though...
No idea what they are, but they're doing the nasties.