I would love a lightweight core that doesn't suck water - infact I made Goats for a couple of years with extruded polystyrene, but even these were too heavy compared to M grade polystyrene.
If the expoxy/styrene boards feel too floaty, the answer might be to get the volume right. Don't epoxy surfboards require slightly less volume?
Personally I want my next 'comp' board to be around 6 kilo's (without fittings). I did the whole weight thing with goatboats for 20 years and light is the go for high performance surfing in average surf.
However, if someone could make me a PU board under 7 kegs I would be more than happy to ride it though