How about an acoustic modem for use underwater?
Whilst high speed internet access is of limited use
beneath the waves, good voice communication between
the surface and more importantly between individual divers is likely to
be of interest to both the commercial and sports diving industries.
Going digital with each diver having a small computer not only provides
the option of multiple / switchable communications channels for addressing a group or private chats (encrypted if need be)
but could also provide telemetry for bio-medical purposes or
equipment status.