  During the last years, it has become increasingly clear that smartphones are
valuable tools to be used almost everywhere. Until recently, a place that still
resisted smartphone onslaught was the aquatic media. However, nowadays, many
modern smartphones are waterproof and the performance of their microphones
results sufficiently adequate to employ them as hydrophones. This capability
gives rise to several interesting applications. Here, we describe two
experiments in underwater acoustics which require two smartphones -- at least
one should be waterproof. The first experiment consists in a simple
time-of-flight measurement of the sound speed in water and the comparison with
the corresponding value in air. The second experiment deals with the acoustic
location -- or ranging -- of a distant object by comparing the time it takes
for the sound to reach the object travelling in two different media (air and
water in this case) with known sound speed.
