Is the water underneath Europa's ice cap potable? I read this question on Worldbuilding.SE, and figured that the astronomy site would have answers too, particularly for the specific example of Europa. The idea is that Earth's oceans are salty because rain falls on continents, and while the rain makes its way to the sea it absorbs minerals and salts from the land. That piles up in the oceans, and that's why you can drink river water but not sea water.
Europa has no continents; as far as we know it's an ice cap of a couple tens of km, then roughly 100 km of liquid water, and only then something rocky that might contain salts.
Does that mean that the mentioned liquid water is likely to be pure? Or at least pure compared to the Atlantic Ocean? Or do we not know?
According to this 2007 paper, the current research as of the time of their own research had a huge range in possible concentrations of $\text{MgSO}_4$, magnesium-sulfate, with over four orders magnitude (approximately times $30,\!000$) differences between the extreme ends of the predictions.  It conducts its own analyses and near the end of the paper makes some analysis on habitability.  They say (with slight formatting modifications for units by me):

If the ice and liquid water layers on Europa fall within
the limits of Fig. 2 (A = 0.7) then, by standard definitions
of “freshwater” environments on Earth [broadly meaning
$&lt;3$ g salt per kg
H${}_2$O (Barlow, 2003)], Europa’s ocean would be a
freshwater ocean, though admittedly more salty than most terrestrial lakes. Indeed, in this case, the putative global ocean of
Europa could be more like the mildly saline environment of
Pyramid Lake, Nevada than like the Earth’s ocean. While the
drinking water regulations of the U.S. Environmental Protection Agency recommend no more than 0.25 g of sulfate per
kilogram of water, adult humans can acclimatize to drinking
water with nearly 2g MgSO${}_4$ per kg
H2O without much discomfort
(EPA, 2004; CDC-EPA, 1999). Animal toxicity (the lethal dose
for 50% of the population) is in the range of 6 g MgSO4 per kg
H2O
(CDC-EPA, 1999), but most livestock are satisfied provided the
total salt concentration is less than 5 grams per kilogram of water (ESB-NAS, 1972). If we assume the low amplitude regime
for our solution (A &lt; 0.8) then it is possible that human or beast
could drink the water of Europa.

However the best estimates for that parameter $A$ they had were from magnetic field observations, which put $A=.97\pm .02$.  In this case the article concludes that the subsurface ocean would then be very salty.  The most salt tolerant organisms we know of could potentially survive in the environment.  However, such organisms evolved into such salty niches from less salty ones, rather than having evolved directly within them.  Current evidence suggests that life as we know it is unlikely to be able to arise in such a salty medium.