I saw a Director run an encounter in Draw Steel. There was a 3-cube-deep pool of mud: effectively water that was also difficult terrain, thus incurring a significant movement penalty.

The Director ruled that, since there are no rules that prevent a creature from falling if forced into water, a creature forced into the pool of mud would instantly sink to the bottom. The Director maintained this ruling when challenged.

This does not seem quite right to me. Does a creature actually sink in water if forced into it? If a party is fighting on a boat atop the surface of a lake (bear in mind that this is a possible encounter in The Delian Tomb), and a PC gets forced into the water, does the character instantly sink to the bottom? Does armor from a kit matter at all here? Does a lightweight memonek simply float, whereas a super-dense dwarf sinks? Does freshwater vs. saltwater matter?