The LM7806 is a rare bird, but available, I found one googling -
http://www.amazon.com/LM7806-Voltage-Regulator-6v-TO-220/dp/B008PS8VYQThe LM7805 is much more common, and a 5 volt supply will work lights almost as well, the main difference being that the lights will last longer. At 1A, they will be trying to lose a little over 7 watts each, so a substantial heatsink will be needed. When the heatsink is considered, it might be simpler and lighter to just use a 6 volt battery. There are such things as voltage regulators that don't act as automatic self adjusting resistors, but they are not yet cheap.
For the 3 volt lighting, I would try to arrange things so that the lamps were in series pairs, making them effectively 6 volt lamps.
The problem with trying to calculate the resistor required is that filament bulbs have a very different value when hot than when cold, so the only way really is to either read the manufacturers data sheet or calculate the current from the known voltage and wattage or measure it. Then you can calculate the value of the resistor to make the full circuit pull the required current off 12 volts. Or if its a fully charged battery, 14.5 volts. Thats where a regulator is useful, it adjusts itself to the correct value for the voltage its outputting.
For 6 volts off a charged battery, 14.5-6 = 8.5 volts. At 1A, thats 8.5 watts. If you want to keep the regulator within its temperature limits, you will be looking for a heatsink rated at better than 4 degrees centigrade per watt, and that is going to be a substantial lump of aluminium to give the needed surface area without fan assistance. And that assumes that there is somewhere to transfer the unwanted heat away from the insides of the boat to the outside world.