Let’s analyse this in a bit more detail shall we.
The aliens come to Earth across light years of space to get water one of the most common compounds in the universe, using vast quantities of energy to use it as fuel, to fight a reasonably advanced technological species (us) on their home turf. The aliens aren't that advanced or else it wouldn't make a decent film.
They pass comets (dirty snowballs), the rings of Saturn (mostly water ice), Europa and other moons which possibly have liquid oceans on their surface – because they can’t melt ice? Just give the aliens a couple of kettles and probably they’d piss off.
By the way I don’t think the aliens were using fusion because there was some nonsense mentioned in the film about sea levels dropping. So how it was supposed to work? Who knows? Because there ain’t much energy to be obtained from water using Chemistry.
Well to be honest the producers did get one thing right, as far as we know we are the only place in the universe to have liquid water on the surface, but that’s almost certainly because we’ve not explored much of the universe yet and not because it is uncommon because as far as we know, Mars does occasionally have liquid water on its surface.
This film was yet another example of film-makers not thinking things through.
And don’t get me started on Highlander Two. Actually now that you’ve mentioned Highlander Two there were lots of problems with the plotting of this film, but the one thing I sat in the cinema thinking but… but… but… at was… To stop the problem with ozone depletion, scientists had put a protective layer over the Earth to block out the UV light. With me so far? One side effect of this was that it blocked out all sunlight. So… all the plants would die and everyone would starve. What a fantastic idea that is.
This was such a good idea (putting a dark blanket round the Earth) that the makers of the Matrix reused it. The thing is the makers of the Matrix could have got round it by having the reality of the Matrix an artificial reality within real reality and said, ‘of course you could tell it was an artificial reality because the science made no sense'.
So any film producers watching this, before you decide on a plot for a block-buster please run it past somebody with an O or Standard Grade in a science.
Tell you what, speak to me and I'll tell you any holes I can think of in your scenario for free.
Oh by the way as far as science making no sense – see quantum physics, it makes no sense to me.