I'm also working on a game that uses lots of water, but probably completely different. I can work with the fluid .cap that you graciously uploaded, and I made some changes to it (removed the darkening over time), but I'm still wet behind my ears (pardon the pun) to get that ellipse collision going on.
Can I get another .cap with more liquid stuff please?
Here you are: fluid2.cap
But remember to act responsible, don't drive today
... Also, does having two sprites = one water drop slow anything down at all compared to having only one object and still using ellipse? The faster the better
Hmm, well, it is slower to have 2000 sprites compared to having 1000. But Construct is pretty fast when it comes to sprites, so I don't think that you will gain so many frames that it makes sense to forego the fluid effect...
EDIT: On the other hand, it would be very helpful to be able to set the radius of the ellipse collision independent of the sprite's size