The universe is really “filled to the brim” on the very large macro level; you look at a galaxy and it looks very populated, but the actual distance and influence between individual stars in minuscule. Between individual galaxies even less. Matter in space is very minimal. If you’re referring to the fabric of space-time itself, then that is everywhere. But hydrogen, the most abundant element, is sparse. Free energy is weak — imperceptible gravitational ripples and microwave radiation.

Yours seems to be hypothesis-by-analogy, ie. water is a medium, then why not space?. Analogies are good for conceptualizing things and simplifying concepts, but not for workable theories. As kindly as I can say, I think that is the case here.