[tortoise]

Maybe one of you hard science types can tell me: wouldn't the water tend to boil or evaporate more if the pressure were low?

Not if the temperature is sufficiently low. This patch of ice is in a particularly cold area of mars, deep in a crater that does not get much in the way of sunlight.

The sublimation temperature of ice at typical Mars atmospheric pressure is just under 198K. There are parts of Mars that rarely get this warm.

[Fitzcarraldo]

This shows if the ice is cold enough, it will remain a solid.