If you can you need a wet/dry vac and vacuum as much of the crystals off first, then soak it really well and suck the water out, or use a brush to get the crystals off and then just really soak the seat and then use a bunch of towels and a fan and soak up as much of the water, not just on the top but push down and soak it out of the cushion.

The problem is that when you use warm water it dissolves the salt and goes down into the cushioning. You can't see the salt anymore and it looks clean, but the foam underneath is still wet with salty water and as it dries it seeps back up into the seat cover and dries salty again. You need to either suck it all out with a vacuum/steam cleaner or dilute it and absorb it out enough that it's no longer salty water.

Cow. Or Elk. Theoretically should work.

vinager dissolves that quickly