Hey, that's a super good question!

That much water will actually collapse on itself because of its own gravity (all matter makes gravity). It will get much smaller because of the pressure as it collapses, and it will get much hotter at the center because anything you compress gets hotter. This is because the molecules have come closer, and so have sort of fallen toward each other. This means they're moving faster, and that is what heat is - molecules moving fast.

Eventually, they will be moving so fast, that the hydrogen atoms in the water (which is H2O after all) which will be slamming into each other from time to time, will begin to fuse together to form Helium. Sort of like if you slam oranges together hard enough they could smash through each other's skin and form one bigger mass. When they fuse, they release even more energy (a whole lot of energy), which makes their neighboring atoms move faster, and drives the process on. Why is energy released when fusion happens? Because not all of the hydrogen becomes the helium - a tiny bit escapes - just like a few fragments of orange which will splatter. This is the energy which is released - because ultimately, the matter around us is energy - as einstein showed by his [math]E=MC^2[/math].

Eventually this will make the water 'ignite' into a new Sun itself. That is, it will get so hot that it will begin glowing. This is how our Sun works too - by fusing hydrogen into helium at its core, getting hot, and eventually radiating heat from that.