Sequels are always bad. And we would have to make another thread for another film.
Not all sequels are bad. Take Star Wars for example, the original trilogy, at least.
Staying in the ships is probably one of the best idea to get away from zombies, as long there's no idiot smuggling infected people aboard.
And I'm sure during the early hours/days of the infestation, the navies task forces would have been deployed. Not to mention all those submarines, from hunter class up to those big ballistic-missile nuclear submarines like the Russian Typhoon-class and American Ohio-class.
From what World War Z book mentioned, many survivors that escaped using ships gathers near islands that are not connected to the mainland.
And if the island have zombie infestation, their number would be manageable to confront. And from those islands, the human can setup command bases to organize some plans to retake the mainland.
Islands like Guam would be most suitable. This is because it have airfield where B-2 bombers can land. Plus the naval base there as well.
If it was me, I would order the retake of the mainland continent per continent. No need to spread our forces too thin.
Make a beach head, and from there, we sweep the place, literally, of any zombie.
Y'all have to remember, we're fighting zombie, not some country with military and war machines. So all those complicated military tactics and maneuvers would not be necessary. There wouldn't be any infiltration mission. We just gonna do a frontal attack. A slow, steady frontal attack.
That way, we can make sure no zombie can slip through the line, and we don't have to worry about zombie attack from our rear.
Let say that we wanna retake American continent. We land at the bottom tip of Argentina/Chile, then slowly moving up to Canada. Time is no longer an essence. No need for us to rush.