If you use lighter-gauge wire you will have charging
loss (and I'll explain why), but heating
with a 20A current will not be a problem. For example, building wiring codes allow you to carry 20A in a #14AWG wire when it is in free-air, and 15A when it is bundled with other wires in a conduit. A #14 wire can carry over 150A before the wire melts (the insulation
goes much sooner, of course).
You can't ignore the voltage loss you will get in the wire (and as was mentioned, the distance that counts is the total length of both wires). Your wind generator
will put out a certain current into a particular voltage. It will deliver less current into a higher voltage, and more current into a lower voltage. This voltage is determined by the voltage of the battery being charged, and the resistance of the connecting cables
(and the current flowing in the cables). The more wire resistance, the less current the generator
will deliver to the battery.
Once the battery has sufficiently charged to where the regulator reduces the charging
current the cable resistance becomes less important. But it is during the maximum-charge portion of the charging cycle where you get the most bang for your buck.
There are different regulator types that can make the analysis even more confusing, but none of them can eliminate the effects of wiring resistance.
You can no doubt go lighter on the wiring, and everything will still work. You will pay a price
in longer charging times, and reduced power available on your boat. A rough estimate isn't too difficult to make, perhaps the loss won't be enough to matter to you.