Say, for example, that youre building a mid-size boat. The skeleton is aluminum, and the outer hull is stainless steel. It has to be water tight. How do you attach the hull skin to the frame if you can't weld it?

I've heard of this before. I think the coast guard welds aluminum to steel in thier boats all the time. DMC sells a transition joint. Not sure if it's something you can find easily though. Web site is www.dynamicmaterials.com. They weld (and they say it's a real weld) all kinds of metal together that people say 'it cant be welded'.

Not a direct answer, but there's something that needs to be pointed out here. If you put two dissimilar metals in contact with one another, and expose this combination to an electrolyte (such as salt-water, or in fact, water with any minerals at all dissolved in it) you get, in effect, a shorted out electrical battery. The result will be that one of the two metals will corrode much more rapidly than it otherwise would.

I hope you're not planning on using this boat in the ocean. In fact, I hope you've got a lake somewhere that is full of 100% pure distilled water, and that that's the only place you intend to use this boat. Either that, or I hope you don't intend for this boat to last very long.

HG, I asked a friend who knows all about welding, and he says that it is possible to weld stainless steel and aluminum. Here's a link to a website that says the same thing:

http://www.muggyweld.com/

From the site:

"Super Alloy 1 is a multi-metal, low temperature solder with a melting point of 350o. Its unique properties allow you to join copper, ALUMINUM, brass, pot metal, white metal, zinc die cast, STAINLESS, galvanized, pewter and steel in any combination. This rod allows you to join metals such as STEEL TO ALUMINUM, which welding instructors are STILL teaching can't be done."

I know this didn't answer your question, but I hope it will make your question, good as it is, unnecessary. IOW, I hope it solves the problem.
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Hungry Guy, you're a sweetheart! Thanks. It's really enough for me to know I helped - ratings be damned.