As far as making a bridge goes, isn't it easier to use the ramp tool and target both shores?
You can try to store a height value and make a bridge out of that, but what if one shore is rather flat and the other is on a hill? Odds are either the bridge end of the flat shore won't be on par with ground level or the cliff side will be dug in. With the bridge tool both ends match the correct height.
I could be wrong, but I think the way height works on all maps is zero is sea level, so negative always means submerged. Sometimes the average ground level is significantly higher than on other maps so you have to dig more to reach water.
Is there a modifier key to snap to height zero when terraforming/placing a building?
I don't have a great way to make stuff at height zero on water. I try to estimate the number of elevations I need to set the terraforming to. Since the sea floor tends to vary in depth, no single +X elevation level from the seabed seems to function.
Is there a use case where you need to make several unconnected platforms at the same elevation other than snapping to zero? That's the only use case I can think of I that I need to store a height value instead of using either the ramp or the level tool.