In the context of potable water, I had never heard anyone insist that seawater was not "tainted water" until 50 seconds ago when reading the comments and questions on this very SE question.
Asked in any other context, practically everyone would agree seawater would be a drink if it weren't tainted, and would be again if it were purified.
TL;DR: Yes, sea-water can be purified with the spell. The onus of burden is on others to argue otherwise.
Is your game's magic a beneficial supernatural force or a trickster genie?
Many people have drank sea water before. I have. Many people would love to drink it all the time, "If only I could purify it!" they would say. This is exactly what the spell seems to be expressing.
So when your mages cast spells, do you let them have the intended effects that they rightfully expect? Or is your magic system like that of a trickster genie who puts someone in a safe prison for eternity when they wish that nothing could ever physically hurt them again, fulfilling the wish only on a technicality?
Some people like thinking of magic this way, but if you want that you should discuss it with your group.
I once read of someone using freedom of movement spell, and while it was still active they jumped into the water to get away... DM stated the PC fell to the bottom of the ocean. If magic is not discussed that way ahead of time, that's way uncool.
But if you want to get nit-picky...
Most things are not poisonous in moderation (or are even good for you), and most things are poisonous in too large of a quantity. Iron, copper, vitamins, minerals, WD40, antifreeze, etc., etc.. A drink containing all of these would still be a drink if they were all present in extremely small amounts. Increase the amount of any single thing and eventually it will be poison, even in the case of some vitamins and minerals.
What the spell actually does
Let's just take the spell at face value. The following statement is directly within the spell's wording and therefore is not up for debate: All drink is purified.
"Is water a drink?" That's a given.
Water is a drink, this spell purifies drinks of which water is one, so it purifies water.
Anyone who argues "Water is a drink, but sea-water is not!" has just rendered the spell useless. Water is a drink, but poisoned-water is not, so by that logic the spell doesn't work on poisoned drinks. Diseased water is not a drink, poisoned or diseased food is no longer food, so this spell works on none of them... we just rendered the spell completely and utterly useless.
Trying to be an obnoxious rules lawyer simply does not work in this case. The spell does exactly what it says it does.
Possible Alternate Interpretations
I can think of a few interpretations which could change this, but they would all require assumptions not stated in the spell.
For example, one could argue "a container full of nothing but poison, which contains no water molecules at all, but which much later on has had enough hydrogen+oxygen atoms combine to form water within it that there are trace amounts of water, is obviously not a drink. This answer suggests it is a drink just because there is water present."
Since "a cup of drinking water with a few drops of potent poison added" is obviously what this spell was intended for, the absurd extreme of "a container with trace amounts of water" could be perceived as a challenge. There are several ways to look at it:
- Was it initially intended to be a drink?
Water+poison: yes, it works. Poison+water: no, it doesn't work.
This option requires the magic to know of the previous state and/or location of the bits of matter that make up the fluid or object. Whether this should be a criteria should be based on house rules, not RAW rules-lawyering.
- Is there enough water to be considered a drink?
Water+poison: yes. Poison+water: no.
Whether there is a percentage cut-off to "How much poison before it's just considered a pile of poison instead of food or drink?", and if so what that percentage should be, should be based on house rules rather than RAW rules-lawyering.
- Does it contain any amount of drink in it?
Water+poison: yes. Poison+water: yes.
Resolves the situation easily but cannot be considered part of the RAW reading as it would let the spell do things it obviously wasn't intended to do. Example: there's a barrel of poison enemies are going to dump into the lake to pollute the whole area, but you just pour a few drops of water into the barrel and cast the spell - does all that poison simply disappear because of the few drops of water?
Interesting implications, maybe you want them, but it would be based on house rules.
Yes, it works on sea-water.
When your players try to take advantage of this when they enter a salt mine with a leaky ceiling and they want to turn the slop on the floor that is 70% salt and 30% water into pure drinking water, then go ahead and revisit the issue. But note that even then it's not a RAW rules-lawyering issue but instead is a subjective house-rule issue.
There are corner cases, as mentioned in my previous section, where the spell becomes tricky to judge. But the specific case in your question is not one of them. People generally consider sea-water to be a liquid that they would drink if it were purified to remove the risk of salt-poisoning. The spell says it purifies drinks.
Unless a compelling argument is made to the contrary this would be the default reading.