-snip-
I designated the bottom floor as a water source, however I still get the same error.
And in the meanwhile I've been trying to get my dwarves to use magma safe materials to make a couple floodgates and mechanisms, however even though I put a large stockpile of orthoclase RIGHT NEXT TO the mason's workshop, they go far out of their way to pick up non magma safe materials from my regular stockpile.
Welp, I'm stumped on the water one, so hopefully somebody else can jump in with a solution.
As far as the magma-safe stuff, you can accomplish that by either A) using the burrow system to restrict your masons to taking jobs/using materials near the desired shop and stockpile, or B) using the 'z' Stone menu to disable all the other stones on the list. The downside of A is that it requires working knowledge of the burrow system and, if you're anything like me, you tend to forget about the dwarves you exiled to the burrow even after they produce the goods. The downside of B is that every dwarf in your fortress will only use the allowed stones, so if your magma-safe stone is in limited supply some random craftsdwarf might use it up before your mechanic can. That isn't much of a problem for me, since I habitually do this just to ensure uniformity in my masonry.
The third option (and the way it was done before the burrow system) involves locking the desired professional in a small room with the workshop and stone until the job gets done. It still works, but is somewhat crude and micro-managey. Properly managed burrows can save you work over the long term, and option B works well as a "fire and forget" option.
EDIT: NINJAS!