What happens if...

[Morwen]

I can't see how Sauron could "summon" them.

In "Shadow of the Past", FotR, Gandalf says the following in discussing Gollum:

Mordor draws all wicked things and the Dark Power was bending all its will to gather them there. The Ring of the Enemy would leave its mark, too, leave him open to the summons. (emphasis mine)

Earlier in that same chapter Gandalf states that Sauron has gathered the Nine (rings) originally given the mortal men to himself.

If these rings have marked the Ringwraiths in the way that the Great Ring marked Gollum then they are similarly open to Sauron's summons, especially as he holds Nine rings and they have no independent wills of their own. As to how this summons worked, I thought that it might take a form similar to the mental communication that Gandalf, Galadriel etc display in "Many Partings".

It might be torturous for them, but is their fear of the water stronger than their fear of Sauron? I think they could negotiate the ocean; it's just that they'd have a really bad time!

I agree. I think that fear of their Master/a direct call from him would trump the fact that they happened to find themselves in water.