It's my understanding that Mantas have a stinger of some kind on the very tip of their tail. So yes, that would be very scary indeed. First your boat busts to peices then you get a stinger in the eye.
I was looking at some underwater footage of a Blue Whale and thinking about the time that I crossed with one. Man, that was a nervous moment! We passed right over top of its tail area, after feathering up long enough to allow it to cross.
(we were on port tack so we were burdened with having to yield rights to the whale)
You can see the footage of a Blue Whale
right here. Click on Channel Islands, then click on Mammals, then click on Baleen, then click on Blue Whale, then click on see movie, then choose the connection speed and type of viewer to use.
GARY