You'd need a 3 wire + ground for a 6 cc't subpanel. You will need to ground the subpanel at the boathouse according to your local code. The ground and neutral buses must be isolated in the subpanel. How far from the main panel to boat house? That will determine the feeder size based on capacity + distance. I'd go with a 60A sub fed by 6/3 copper if less than 75'. If more than 75' add one larger size wire for up to 200'. There is a good FAQ on this site about subpanels. Look it up in the FAQ's. Brand of sub and breakers is really up to you. I like Square D QO, but that's me. Every receptacle in the boathouse must be GFCI protected. I'd use THWN individual wires in PVC conduit buried 18"-24" depending again on code in your area.

Thank you bigbird.

Since the wire already exists, is there a single circuit solution to this? The distance is about 75 feet. I'm trying not to have to dig. I'm hoping to find a way to have a couple of lights in the boathouse.

Cheers.

With the current 12/2 the best you could do would be 120V at 20A which you already know about. You could run the 12/2 as 240V only, but that won't help. So you would need 3 wire to meet your requirements.

Why do you feel you need a sub panel? Use the current circuit as a single circuit and feed your devices in the boathouse.

OK. Great idea. What would be the best way to split the single circuit to allow 2 light switches and a series of wall plugs? Is there another type of junction box? I'd like to avoid pig tailing too many wires together.

i would mount a round or octagonal box on the wall and feed it with the incoming wire and have two cables coming out, one for switches, other for wall plugs.

The junction box will have 3 wires on a wirenut, the wall plugs can be wired using all the screws to feed the next one, two pigtails in the switch box for two switches.

Route the cable from one box to the next as needed. No need for a main junction box.

ANSWERS and THOUGHTS:

1- What we need here is a 240V isolation transformer with a center-tapped secondary... Then we've got 4800 "watts". We send 240 down our two wires, transform it, treat the secondary side as "separately derived", CT being neutral.

2- There IS such a thing as a 120V subpanel, it's just a really uncommon and rather dumb idea.

3- Aren't there certain conditions under which we can use the ground wire of a feeder to a seperate building as neutral? Maybe he meets these requirements?

4- If he's already got 20A @ 120V and just wants basically one circuit worth of stuff, why not just leave it as-is? Heck, I've seen people do what he's talking about without wiring anything at all!