What fin are you using?
The stock fin that is supplied with the Serenity is very "bitey" (for lack of a better term)
and it's pretty difficult to jibe the board with that fin. When you get to downwind, and the
fin goes from biting on one side to biting on the other side, it hooks up really hard and fast
and that's what may be throwing you off.
Try a smaller fin (maybe 60 cm or so) and I think you'll alot better results.
But your reference to sailing clew first indicate you are trying to jibe the board like something much faster and much shorter.
The technique I used on the Serenity was adapted from racing longboards.
I think it's called a "flare jibe".
What you need to do is rail the board so it turns off the wind (very difficult with the stock fin) and then lean the rig well out to the side to "drive" the board the rest of the way around.
In order to make it a flare jibe you need to really tip/lean the rig way over to the windward side to drive the board around and past dead downwind.
Then stand the rig up and flip it.
A clew first jibe just doesn't move the pressure far enough off the side of the board so the board can rotate around the fin.
You can step back some, but that pretty much a guarantees that you'll end up in the water. (Eddie from WS Mag was really impressive with his sink/flare jibes at the first Serenity demo in the US a few years back).
Actually when you think about it, if you go clew first, with the really long Serenity waterline, the clew first is pretty much preventing the board from turning past down wind
Try flaring the rig out to the side and I think you will have a lot better luck and stay dry alot more of the time.
Hope this helps,