I don't know! Let's find out.
So, Clericsdottir. You cast Sanctify, cool. So how does Siggrun, the Secret in the Deep relate to making sea water drinkable?
"...you know they mine salt, right?"
Oh! Okay, so-
"They mine worse things, too. Just a grain left in the water can kill a whole crew. Sea water is baby times for Siggrun."
Now let's find out how we found out.
When you find yourself in a place as a GM where there's a problem in your world that you think a spell could solve, even if the text of the spell doesn't perfectly match up with it: great, go for it! Offer an opportunity that fits a class's abilities. Cast a Spell is always a risk for a caster, and they'll be taking that risk to get it done.
When you find yourself in a place as a GM where there's a problem in your world that one of your players thinks a spell could solve, even if the text of the spell doesn't perfectly match up with it: great, let them go for it!* Be a fan of the characters. Cast a Spell is always a risk for a caster, and they'll be taking that risk to get it done. And while they're getting it done, you can get some more details about their relationship to their god and how their god works - you know, the thing they had to make up to even be The Cleric in the first place - by asking how they're doing it.
Asking how the characters are living out the move their players are making is an important part of Dungeon World, and maybe an unusual part if you're not used to the system. It isn't done to try and disallow a move on a technicality, but instead, to help everyone be clear and expressive about the fiction. Because when Clericsdottir goes to cast Sanctify and rolls a total of 5, you the GM are on the hook for what happens next, and it's a lot easier to make that path forward if you have the whole story going into it.
Oof. So, uh, Clericsdottir. Mithralar - the Unbounded Ocean, the Breaker Unbroken? - is one of them there jealous gods. And for all that Siggrun goes "I'm holding you up too, you know", it never takes. So this water is no longer mundanely undrinkable. And until you propitiate Mithralar, or until they get bored with you, that goes for any sea water you set your hands to.
*The exception, of course, is if there's something in your prep that you owe it to honesty to deploy here, such as if you've prepped for an adventure of hard-edged sea survival and you know that Mithralar, the Unbounded Ocean, has a permanent snit fit as regards sea water. Or you feel it should be covered by a different spell, like a higher-level one or one from a different class. Or you feel it's the provenance of Ritual or Divine Guidance, the more free-form way The Cleric and The Wizard do things. In those cases, go ahead and tell them the requirements or consequences, or offer an opportunity that fits a class's abilities.