Driftwood isn't for guitars. It would require so much work and time (and a good kiln) to transform a piece of driftwood into a useable wood suitable for guitars that it is not feasible.
A number of years ago there was a company that set out to reclaim logs that had been dumped by a barge on a river in I think Oregon or Washington. They'd been in the cold water for a century and pretty well preserved. One of the things they were talking about doing was using the wood to create some acoustic instruments as there's been speculation that the wood used in the Stradivarius violins had been reclaimed or spent a significant amount of time in the water before being used.
As long as it's properly dried I can't see it as an issue. It'd be a fun project.