I think he just wanted to heat up the water and form a spa. You know, like hte mist generator creating pleasant thoughts, except this time its a bath heated by magma underneath (kinda like how the Romans heated their baths by installing hot coals underneath them).
I will second the notion of pre-designing your fort. In the beginning, you probably shouldn't go too crazy with general layouts. You'll want to explore and try out a few things to see how they work, and then use that experience to tweak your designs. But when you find one that works for you, by all means, save it. I usually like to spice things up, because it can get stale going through the same routine over and over and over.
However, the main thing you need to think of before heading out, is what your goal for said fort will be. In the case of your Magma-spewing Dragon-cannon, you should go ahead and model it in 3D... or hell, use Legos if you still have some in the closet XD I actually did that when buiding stuff in Minecraft. However, that is an advanced project that will likely take years in-game to build, so you'll want to focus on getting the basic fort set up and pump up its population in the first few. What industries will you focus on? Will the dragon be built entirely out of metal? Green glass? Gems for the eyes? Will you CAST it out of obsidian, then engrave parts of it? Will your dwarves live under the fort, or will it be large enough for them to live inside? Will you build the interior "skeleton" first for your dwarves to live in, or will you permit an underground "minimal fort" they can live/work until the quarters above are finished? Whatever you want, you'll need the hordes of unwashed peasants to actually build it. So are you going to fish for food? Get farms going asap? Plant-gathering and stepladders for an early food bomb so you can dedicate more people to other work for the rest of the year? Do you know what industries you want online first? Is the area poor in wood and you'll want to bring some coal at the start and/or log the caverns? Import cloth and hope you have enough to last the completion of the project, or get clothing industry online with the 2nd/3rd migrant wave? Are you going to get sidetracked getting your tavern/temples set up, having to branch out in industries like pottery to make that cool instrument you want, or just ignore them/set up a basic one for civilian-only use? Will you chase higher level nobles and get the King's bedchambers/throne room set up in the head of the Dragon?
My point is, you need to have a fairly clear idea of what you are accomplishing when embarking on a new location. In dwarf fortress, there are virtually no pre-built templates- you have to dig, designate, and build everything a tile at a time. Its easy to get overwhelmed or lost, as you get a wave of migrants and have no idea what to do with them. You dont' have a clear vision of what your fortress looks like, so you just start dropping things randomly. Its a lot to take in (dozens of professions and workshops, interlinking supply chains, stockpile settings, oh my!). So a little pre-planning and visualization go a loooong way. You can write down a simple checklist to keep yourself on track, or save some templates with notes added for the order in which you'll build them. You don't want to dig out your entire fort in one go with just 1-2 starting miners, after all.