The Ancient Mariner and his fellow crew-mates have hit the doldrums. They've reached that notorious part of the Atlantic Ocean renowned for its sudden calms, storms, and unpredictable winds. At the moment, it's a sudden onset of calm that the men have to deal with. Without any wind to guide them on their course, they are all adrift at sea, unable to continue on with their journey.

There's something unreal about the situation in which the men now find themselves. They're supposed to be sailing the ocean, moving along at a rapid rate of knots. Yet because of the doldrums, they're not actually doing any sailing, or anything else for that matter.

If we were to look at the ship just sitting there, not moving at all, we could be forgiven for thinking that we were looking at a painting of a ship rather than the real thing. That's what the speaker means when he refers to a “painted ship” upon a “painted ocean”. Everything's static, just like in a painting. All is silent, all is still. And the silence of a painting corresponds very closely to “The silence of the sea” that the men can only break with their conversation while they wait for the wind to arrive.