Hmm. I did not think that the temperature of Europa's oceans (below their probable icy crust) has been well-constrained. To the extent that it has, I think the subsurface temperature is expected to be much greater than -50C in many places (the mechanism is tidal heating).

There is a general belief that Europa is likely to have significant liquid water below its icy surface. The phase diagram for water (first image at http://www1.lsbu.ac.uk/water/phase.html) does not allow liquid water at -50C = 223K, no matter what the pressure.

Atmospheric pressure on Earth is about 0.1 MPa, and gravity on Europa is about 1/6 of Earth. The highest pressure in Earth's oceans is about 1000x atmospheric pressure (100 MPa). Across this range (3 orders of magnitude), the melting point of H2O is pretty constant, 0C or a little below (see the linked phase diagram).

My conclusion: liquid water implies temperatures of at least 0C in the relevant pressure range.