There are ways to filter out the salts which allow you to drink sea water.
Filter out? No. The salts are dissolved, not hanging in suspension. There are chemical ways of recombining the soluble salts into unsoluble sediments, which could then be removed, but that's far too much like a chemical-plant, for my liking, and depends on using finite (if available!) resources.
What I'd do is (to be heavily modified, depending on what I've got at hand) stand a small bowl in a larger bowl of sea-water, both placed in a small hole in the ground, with a (transparent or black - each work better than white, but for different reasons) plastic sheet over that that is anchored by stones/similar at the edges (passed over and round-under, by the sheet) with a stone/similar holding the centre down over the smaller bowl (perhaps also with something white between sheet and stone, above that small bowl, perhaps with some water in the dip), and let the sun get to work.
The large-bowl's water will evaporate, a significant amount will re-condense on the sheet and trickle to its downwards-pointing apex, to drip into the central bowl. The optional central covering reduces how much re-evaporation occurs (most of which should just re-re-condense), while the water (need not be fresh) in the hollow should increase the condensation rate beneath the bits of the sheet that it covers.
There's so many ways of doing this, though, and it doesn't need to be efficient if it's giving a sufficient supply for your needs. Despite the complexity of my above description, however, what I gave is a pretty simple method to construct and leave unattended. (Assuming you're not getting wild animals rooting around, strong winds or someone on your territory liable to disturb your setup.) Putting together a proper boiling-point distilling-type kit is probably a better option for quantity if you have the materials and fuel.
(Also, depending on what the water-supply
is, and how well you set the equipment up, it might still benefit from adding purification tablets, but at least it'll be much lower in salts. And of course different survivalist-types might have their own solutions, I'm just giving you what I've used (
not in anger!) and found to work. Albeit slowly in the kind of sunlight conditions it was tried out in.
)