I know little about SMOS, but their sensors do detect "salinity". So conceptually, it would register surface melt differently from a salt water wash. However, SMOS reports on a 35 or 40 km grid (I've read both; I presume it is one or the other, unless it is latitude dependent), so the sensors-to-data-output-system is doing a great deal of averaging. As open water is very wet, as soon as SMOS looks at floes that are 'small' (and not perfectly packed together), it starts averaging (convoluting) thickness with wetness and the thickness output becomes seriously/totally irrelevant (per SMOS literature). As Oren suggested, a salt water wash will occur mostly along the edges of floes, so the salinity SMOS detects from open water might be very slightly exacerbated by salty water on a floe. I conclude with Oren, "negligible", and add "irrelevant after review."