The health dangers depend on the exposure level. You'll want to keep the
transmitter power low, of course. Think of all the smart phones that are
running WiFi transmitters in close proximity to people - we haven't seen
any health warnings about those.
You probably don't want to use a standard WiFi signal - that is designed
for data transmission. (Unless, of course, you want to add some sort of
telemetry to the signal.) The WiFi requires a stronger signal for good
through-put, and the output is pulsed (and somewhat unpredictable when
it will be sent, making it more difficult to locate.) You would use the same
frequency bands (which are shared with microwave ovens and cordless
phones, among other things) but with a modulation selected for optimum
I'm using WiFi as an example of an unlicensed frequency band for which
a lot of equipment is currently available that could be modified for your
purpose. The 5.5 GHz band could probably work as well, with slightly
smaller antennas. A suitable transmitter chip designed for this service
might use an IC 2.5mm square (I have thousands of them sitting on my
desk at the moment) and, if there was a big enough market, the
control circuitry probably could be squeezed into the same package
without making it much larger. A quarter wave antenna is about 2.5cm
(one inch) long at 2.4 GHz, or half that at 5.5 GHz.
But you'll have to test it to see how well it propagates over the ranges
you want, given the typical circumstances in the hull.
The higher frequencies (like microwaves) tend to reflect well off of
walls, and can sneak through small cracks. But they get blocked
by fog, leaves, etc., and don't carry as far unless you are using high
gain antennas. As you go lower in frequency you get longer range,
but more attenuation going though buildings because they tend to
be more blocked by metal framed walls, small windows, etc. They
also require a larger antenna to provide the same angular resolution
to locate them. At some point the wavelength gets too long and
the signal won't propagate through a confined space as well (the
"waveguide beyond cutoff" principle). It's a matter of choosing the
right frequency range for the coverage you need, then finding a
band that is available for your application (either an unlicensed range,
or one that permits your use), and where you aren't plagued by
other interfering signals. You'd want to stay well clear of any radar
bands, for example, because the system could become unusable if
a Coast Guard or Navy ship were nearby.
Lower frequencies probably give you a better indication of "you are
getting close" over a range of perhaps 10m or so, but it will be much
more difficult to determine the direction. Ultrasonic radiators and
detectors might be worth trying - you could use a fairly narrow
filter to eliminate most interference from engine noise and a small
parabolic dish for finding the strongest direction.
And in the end you might want to have a couple different systems
running inside the same piece of equipment - a manual strobe light perhaps,
with microwave DF capability plus some sort of "you're getting close" signal.
There may be other safety features that you want to add to the box while
you are at it.