Hi Bob

The viscosity of nitrogen and methane are less than water at normal liquid range temperatures - "thick" they wouldn't be. Water/ammonia mixture at their eutectic (176 K) is actually quite viscous, like honey, and methanol/water eutectics are even more viscous.

BTW in Ben Bova's "Titan" he has the local lifeforms existing in a mix of water and hydrocarbons at ambient temperatures - can't happen. But you could have a layered arrangement of methane over ammonia over water according to one piece I read (an old study on Titan from the 1970s.) Thus, in theory, an open body of liquid - deep liquid - would potentially be a stable opening into the liquid mantle.