The problem with all these ideas is that, whatever the technology
unless it is specifically made for an underwater environment, you can
count on it getting ruined.  Yes, I said *underwater* -- kayaks roll!
In fact, any instructor who doesn't first teach his/her students how
to do a proper eskimo roll is nothing less than irresponsible.  If you
go kayaking you can be sure your entire head is going to get
completely submerged at some point, and certainly during your initial
training.

You may want to look into the specialized underwater communication
devices that scuba divers use.  This may at least give you a place to
start and a possible source for immersable earphones which may then be
able to be used with some other kind of specialized listening device
that itself can be put in a watertight box or bag.  One company which
makes heavy-duty watertight bags for cameras, personal stereos, etc.
is called Ewa.

Hope this helps,

Jim