  While acoustic communications have been considered the prominent technology
to communicate under water for several years, other technologies are being
developed based, e.g., on optical and radio-frequency electro-magnetic waves.
Each technology has its own advantages and drawbacks: for example, acoustic
signals achieve long communication ranges at order-of-kbit/s bit rate, whereas
optical signals offer order-of-Mbit/s transmission rates but only over short
transmitter--receiver distances. Such a technological diversity can be
leveraged by multi-modal systems, which integrate different technologies and
provide intelligence to decide which one should be used at any given time. In
this paper, we address a fundamental part of this intelligence by proposing a
novel routing protocol for networks of multi-modal nodes. The protocol makes
distributed decisions about the flow in each link and over each technology at
any given time, in order to advance a packet towards its destination. Our
routing protocol prevents bottlenecks and allocates resources fairly to
different nodes. We analyze the performance of our protocol via simulations and
in a field experiment. The results show that our protocol successfully
leverages all technologies to deliver data, even in the presence of imperfect
topology information. To permit the reproduction of our results, we share our
simulation code.
