But then if I embarked on water and modded in a device to turn stone into magma...
You can't do this. It's impossible to mod the game so you dig out a tile and have it drop a 7/7 block of magma instead of water and you would still have to get your dwarves to dig out these tiles (which they wouldn't) even if it hypothetically was possible. If you could somehow get around that, you would have a marginally shallower ocean, a cast obsidian floor, and a sea of magma beneath. The idea of embarking in water and then creating land seems counter-intuitive, anyhow; you can just embark on an island if you want that.
I mean how Embark allows you to access areas that the game prevent you.
make a utility that allows you to work under water.
No, I understood what you meant. Embark Anywhere (to my knowledge) actually
does let you embark underwater; your dwarves simply don't do anything, and if you haven't given them [SWIMS_INNATE] and [UNDERSWIM] they simply drown. So you can embark underwater, it's just kind of redundant to. I don't understand your usage of 'Embark Anywhere but for water' either - The reason EA is used to embark on mountains is that there is
air in the mountains. There is no air underwater.
As for 'make a utility that allows you to work underwater,' you simply haven't been reading what I've been writing. I'll elaborate and emphasize.
Dwarves, when underwater, will do one of two things: If there is an exit, they will swim to it (if they can). If there is not an exit, they will just stand there forever until there is. A tile filled with 4/7 or more water is considered inaccessible even if a dwarf or other creature is within it; you cannot select underwater stone to build with, you cannot stockpile underwater objects, you cannot have your dwarves undertake underwater jobs, no matter how close the dwarves are to the job location. Water pathing is hardcoded to be 'if dry land = nearby, get on land;' there is no 'else' clause, dwarves just cannot do anything underwater.So again I point you to this quote:
You can't. Not accepting jobs that are underwater is hard-coded into the game. There's nothing you can do to change that.