Your requirement is only 5 milliamps of 5 volts from 24 volts? Does it
need isolation? It's not going to be cheap or easy to find a DC-DC
converter that is efficient at that low current.

If you don't need isolation, use a low current linear regulator, like the
LM2936Z-5. US$1.29 and virtually no quiescent current. Put a transzorb or
zener of the right voltage across the inputs, though, along with a series
resistor. I imagine load dumps in the boat's electrical system will make
spikes over the rated input voltage of the regulator.

-bob