For the greater part of my life I have been sailing.
Lakes, canals and also, as the weather forecast calls them:'coastal waters and estuaries'.
Moving water causes waves and wherever there are waves there is a risk of seasickness.
The swell of the open ocean is a completely different movement.
No seasickness for me there. Once in the open ocean 
I feel the slow, comforting swell that moves the ship.
Falling asleep on Europa is in no way a problem for me.