What happens if...

[Mithalwen]

Listen, strange minions lyin' in ponds floatin '
or not is is no basis for a system of justice! Supreme executive power
derives from a mandate from the masses, not from some farcical aquatic
ceremony!

[The Might]

I can't see how Sauron could "summon" them.
They would try to return to Mordor, as they did after the event on the Ford of Bruinen
But I wonder how water would influence their powers. We know they hated water, and Tolkien was aware that the way they managed to cross the dangerous ford at Tharbad is more or less a plot hole.
So what would they do in so much water...?
I don't really know.

[Eomer of the Rohirrim]

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!

[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.

[The Might]

Sorry LjósÁlfr, I believe I initially understood your point wrong.
I agree with that, now that Morwen explained it.
I first thought you mean "summon" as in magically teleport them to Mordor or something like that.

[Thinlómien]

Originally Posted by The Might

I first thought you mean "summon" as in magically teleport them to Mordor or something like that.

Can I guess? You've played (a lot of) computer games in English?
(I speak from experience, I used to have a similar definiton for the verb "to summon" in my head some years ago since I had played too much games such as Age of Wonders and Heroes of Might and Magic... )

[narfforc]

If they floated would it be possible to make a bridge out of them.