As far as I'm concerned, it's a big money grab, it's the same foam you can buy to package items for shipping, at 10 times the cost.
Sand coves are fine, if you want, tape some 6mil vapor barrier against the lower wall and wrap it onto the floor in an L and build your cove on it, this will ensure the cove can't wash out if you have a leak near it. - that's what I did and it's great.
Happy walls and happy bottom are just that because the manufacturer is really HAPPY when you buy it, if you want, use blue or pink styrofoam on the bottom, don't waste your money on the stuff the pool stores sell.