Yes, a submarine would keep floating all the way to the surface if it was slightly bouyant. Or it'd sink all the way to the sea floor if it was slightly denser than water.
It's the pressure gradient around it which causes bouyancy, not the pressure.
The pressure gradient is constant with depth, even though the pressure itself changes.
The pressure gradient is a function of the density of water. Changing the density changes the pressure gradient, and changes the bouyancy of things submersed in it.
Regarding the thermometer. You're mistaken. Water is incompressible, so when you apply pressure it doesn't increase in density - at least not significantly. However thermal expansion does cause a significant density change, and this is the effect that's used.
We can see who's right quite easily:
You say that increasing temperature will increase the density and cause more bubbles to float up.
I say that increasing the temperature will decrease the density and cause more bubbles to sink.
Which way do they go in real life?
