Don Casey's Complete Sailboat Maintenance Manual goes into great detail on voltage drops, calculating the correct size of wire, and so on.
There's a simple calculator here :
Voltage Drop Calculator Genuinedealz.com
But basically for running wires in your boat, the following rules apply
16 gauge is the thinnest allowable on a boat
The breaker should be sized to protect the wire, ie trip below the wire's rated current
Rated currents vary with where the wire is (see the table at the above link, wire is derated in engine compartments)
The wire should be sized to give less than the allowable voltage drop, based on the current drawn by what you are running off of it
You can also place fuse holders near to the device (Blue Sea Systems make some nice ones)
ABYC says that voltage drop for critical devices (VHF, GPS, etc) should not exceed 3%. For others, 10%.
Fuse holders protect the device, breakers protect the wire
For wire runs I prefer boat cable, which is 2 core with an extra protective sheath. Red and yellow are the approved colours now for DC + and -.
I usually keep a good length of 2 core, 14 guage, red and yellow, boat cable on the boat, which suffices for most jobs.
So let's say we have a VHF radio that can draw 6A. Using the calculator above, and entering 14 gauge wire, a 10ft wire run, 12V DC, we get a drop of 2.59% which is just acceptable. For a 20ft wire run we have to use 10 gauge wire to stay below 3%.
14 guage wire is rated for 35A outside the engine space, so a typical 20A breaker will protect it fine.