Exactly. Who would they want to attack? Rand? How could Demandred possibly the Sea Folk against their Coramoor?

Only the Seanchan are their enemy, and the worse they can do is destroy their fleets before the much better trained damane and the huge and impressive land armies wipe them out. The first attempt to attack the Seanchan didn't exactly turn out well for the SF...

Odds are the Sea Folk are soon to get the bad news that the Coramoor asks for all the Windfinders to leave behind the ships to join the rest of the Light on land for the LB. He's going to be far more polite about it then when he sent Logain and left them no choice, but essentially it's what Rand wants of them, I think.

I suspect it's part of their prophecies. Something tells me from their actions in the series that they have verses foretelling some sort of return to the Land before they go back to ruling the seas, stronger than ever. That's what they think they're preparing by securing patches of land for trade (those are obviously Travelling grounds... and they won't be best pleased with Elayne's deal with the Kin... they planned to be the ones exclusively offering Travelling-based Trade and Travelling services. Elayne knows that.) I think they don't realize they'll be asked to abandon their ships and islands to fall behind the Coramoor at the LB.

That, not any "LB at sea", would be the real challenge to the SF culture, equivalent to having the Aiel abandon their spears. The Pattern has even seen fit to weave out the Amayar they were bound to protect...