Terror on the seas!

1,000 to be rescued from stranded cruise ship.

I see three scenarios here:
1) PCs go to investigate this uncharter waterway obstruction only to find an island that points to Things Man Should Not Know . . .including where and when Ryleh will next rise.

2) Some sort of alien got aboard the ship and was killing/infesting the passengers. The military is trying to clean this up, but need the PCs help.

3) Something under the waves, say Namor or one of his enemies, has declared war on the surface world and this is the first attack in that war.