To those have a Prius already

nope

 

It depends on what you mean by flat. If you move the seat all the way forward and take the head rest off then you can lay the seat back all the way.

 

A shorter board should fit in there with the hatch closed. If you have a long board, you're better off with a roof rack if you don't want to drive around with the hatch open. The front passenger seat is able to be almost flat if you scoot it all the way forward and recline the seat all the way back. I never got the chance to test the ease of removing the headrests, but if the Prius is like every other Toyota, the headrests should be easy to remove.
P.S- I do realize that the thread is titled "To those have a Prius already", but I specifically checked to see if you could fit a board with the seat reclined when I did a test drive.

 

just push the button used to raise or lower the head rest and pull the headrest out of the seat back. Had 10 foot mouldings in the car with room to close the hatch.

 

There's a few inches over 8 feet carrying it this way, more if you become creative:

 

I have been surprised at what I have been able to haul in the Prius... I don't know the first thing about surf boards, but hopefully one of these friendly folk helped you

 

I just measured my '04. I did not put the seats down, but I measured from the lip of the open classes container on the dashboard (not the one above the windshield) to the back of the cargo area. It's hard to tell exactly where the closed hatch would be, so figure a possible error of plus-or-minus 2 inches. The length was 104 inches. That's 8 feet 8 inches.

I think, but cannot be sure, that with the seat down the board would slide under the glove box a little bit, and you'd gain a few inches over my measurement.

For anyone dealing with something long and flexible (such as the mouldings Frank mentions) you'd definitely gain a substantial extra distance by reaching all the way under the dash to where the passenger's feet would be. But just eyeballing it, I think that'd be out of the question for a surfboard.

 

Yes, it does and yes you can.

You move the passenger seat all the way forward, remove the headrest and then tilt it all the way back, it's pretty flat. I hauled some long boards from Home Depot this way.

There is a Prius Club San Diego member that can put their surfboard in their Prius. I don't know how long the board is but Sev MacPete would know. I thought he said it was a nine foot board but you should ask him. I would think you'd put both back seats down and put the board in under the glove box in the passenger seat area and then it would be slightly diagonal to gain a few inches.

It's like the Moebius car. I'm amazed at how much I can fit in there. It's like it's bigger on the inside than it is on the outside.

 

You mean, like a TARDIS? Hmm, a TARDIS is blue, my Prius is blue.