Please take a look at this short video and tell me what you see

Hi all, I assume this is the correct place for this kind of thread.

anyway, on another unrelated forum I hang out, a guy posted a single picture of what did appear to be "something" big swimming under his boat, but now he has posted a short video of it, there is a bit of disagreement as to what it actually is. I figure someone here might want to take a look and tell me what you think it is.

Something dark appears at around the 4 second mark toward the top right of screen and appears to move across and below the guys boat. Not the stand up paddle board, but the black thing under him........that's the giveaway as to what I think it is. The argument is whether it is a swimming "thing", or simply the shadow of said boarder being distorted by UWA gopro video.

what say you?

http://s235.beta.photo​bucket.com …fish_zpse9940d2​b.mp4.html

My 2 cents says I see the shadow of the boat on the bottom going over a plant of some kind.

Some guy in a Hobie Adventure Island, all I see is weed.

captured: you didn't see that black thing that seemed to go under the rear of the hobie? or is that what you are talking about?

btw, nice part of the world Lake Cathie, I know a couple of people up that way

My opinion is that is not the shadow from the plank, the thing looks darker than other shadows in the video. Also, it looks like the boat it`s passing over it rather than it swimming under the boat. In other words, it looks more like it`s standing on the bottom of the sea and not moving too much. I may be wrong of course, especially if the water is not that shallow.
Perhaps a stingray?
What do you suspect it is?
And could you post the link to the photo the guy posted?

looks like seaweed to me

It's the weed, man...

Whatever it is it doesn't move the boat moves over it ... I agree its a weed or stump something like that