The surfboard analogy keeps coming up. I don't think it's appropriate.
A boat is hollow inside so the engineering considerations are much different. Any time you use a foam core there has to be an increase in the glass laminations to make up for the properties that foam lacks like tensile strength and stiffness. It's almost impossible to keep weight down if the epoxy content of the boat is increased.
From a construction standpoint foam, being much more limber, needs a much more inclusive mold to support the strips properly.
In short, foam is no panacea. If it was you wouldn't have had to ask your qyestion. You could have just dialed up a site populated by dozens of foam builders.

: Hi there, can anyone tell me if there are practical alternatives to wooden
: strips ie foam? I have designed and built an 8 chine S&G which was very
: successful and would like to build a surfski (long narrow racing sit on
: top), and wonder if it would be lighter and faster to build with a ply
: stringer,shaped foam blocks glued on, and glassed. I believe surfboards
: are made this way. Production surfskis are made with moulds which doesn't
: seem practical for a one off. Thanks for any input. Andrew