Due to the fact that most space craft effects have a sort of fire feel to their engines, I highly doubt submerging them in liquid would be good for them at all.
It might be possible with some clever modifications to the nebula effects, but as far as I can tell all nebula use the same basic design with changes to the colours and texture of the effects, I'm not really sure how that could be changed to a liquid effect. Turn the visibility to very low and turn the clouds into bubbles?