Now that's creativity

Creativity = Killing everyone by taking great liberties in interpreting spells

To me, this just raises all sorts of questions about how magic works, mechanically speaking, in this setting. Does this only work on water in a container (which is not really a thing scientifically speaking) because that’s the consensus of what practitioners believe it can do? Was the spell designed like this by its inventor?

If it destroys water you have ions destroyed with the solution so no salt. Because it's a part of the water. If you did destroy the water then you would get sodium and chloride plasma because it's ionized.

The plasma is why the magic glows.
Which is probably fine since, at this temperature, it will immediately convert to solid salt, albeit maybe with some impurities should some ions react with something else.

If I cut someone do they count as an open container?

If someone opens their mouth do they count as an open container?
- Nope, people are tubes, not containers
- Their intestines might, but not their lungs. Lungs are more liek a sponge

i will get to a level where i have as many slots of destroy water as possible, and i will spend the rest of my life casting destroy water on the ocean, until it is completely dry. is that something i'd be able to do, or is that not possible? (i play pathfinder, not d&d, so i don't know)

After awhile, Poseiden comes and kicks your ass until you stop. Live by the magic sword, die by the magic sword.
Assuming a level 20 druid, you'd have 32 slots 1 or above, 2 gallons per caster level so 40 gallons per cast, that's 1,280 gallons per day or however fast you can regain spells. A bit shy of half a million gallons in a year. A particularly long-lived elf could destroy 280 million gallons before keeling over.
Now it depends how big the world you're on is, but if it's earth-like, the smallest ocean on earth is... 4.9 quintillion gallons.
4,900,000,000,000,000,000
So while you wouldn't make a dent, a character with an irrational war against the ocean is hilarious and you should do it anyway.
- So, the problem isn't how much water but how much time?
  Become lich, be immortal, problem solved.
  Now it's a race between you and the heat death of the universe or pissing off some God of the Sea for fucking up their backyard.
- wait wait wait
  could i just kinda put a stick on the mouth of a bag of holding and throw it in the ocean? (the stick is for keeping it open)
  then i could throw a bunch of those in the ocean, and it'll at least help destroy it.
  if this works like i think it does, i'll get something that lets me craft bags of holding, and i'll just spend the rest of every day (after casting destroy water) crafting bags of holding to throw into the ocean
  also, don't worry about age. i am going to become a lich like someone suggested, so that i can just stay there, destroying the ocean very, very slowly, forever.
  and please correct me if anything i said isn't possible!
- Maybe the duid can team up with a lvl 20 wizard that will open a Gate with his 9 lvl spell slot from the bottom of the ocean.
  Dunno how to do the math, but a 20 feet diameter sink in the ocean opened in the sky of another plan for one minute may be faster (in spell slot economy) than lvl 9 destroy water.

"free" shit costs a spell slot, you know?

Free() can destroy the world. Believe me. I know. I am a C programmer.
- This is why I free nothing and only malloc().
  #WhoNeedsMemoryManagement
  #JustDownloadMoreRAM
Shut up and getting castin' wizard. These biscuits ain't gonna season themselves

i can use create/destroy water on blood to get iron for my weapons?

Bloodplasma is mostly water, but blood is only about 55% plasma. Unsurprisingly, your blood actually has a LOT of stuff in it, like red blood cells.
Unless you want to count water in cells, but that would do the stuff that the spell doesn't do.

Turn beer into moonshine!

I cast destroy water on the fluid lining of his brain.

That would fall under the first part of this meme. You cannot see the fluid lining of his brain unless it is exposed in which case I feel like there's a far easier method of murder.
- You cannot see the fluid lining of his brain unless it is exposed
  I would like to submit, as a counterpoint, the Ring of X-Ray Vision from the 2014 5e basic ruleset.
  Soooo... Destroy Water, please, Mr. DM.
- It wouldn't be (instantaneous) murder though.