I’m reading this book about a zombie virus used to kill US officials on a ship and I wonder if you’d agree?

I find most zombie stories to be severely lacking in logic, so I'm probably not the best person to answer.

But yeah, if killing the US officials on a ship is all there is to it, then I agree with you: there are other, easier ways to do so.

On the other hand, I can think up a few reasons to use a virus instead of a poison. Maybe you wanted to send a very specific message? Maybe you'd like to test out your virus in a controlled environment before releasing it big scale?

Or maybe... and this is probably what I would go with if I were to write a similar zombie story (which I won't):

You wanted to play a bit and see how the US would handle it: You poison someone and they're dead, end of story. You turn someone into zombies and suddenly the US will have a problem on their hands. They might be forced to kill their own people, which isn't going to look good. If there are other people on the ship, then the zombies will post a threat to them, then what do you do? It gets even more tricky if there's even a remote possibility for a cure. How would the US media and public react if you killed all your contaminated officials to ensure the safety of say the non-US staff and four Iranian scientists?

Jaquez, my guess is the author found the concept of a zombie virus a lot more exciting to write than boring old poison, mutiny, or sinking the vessel. (Or he hoped the readers would.)

With a zombie virus, the work feels more current this-could-really-happen. Plus you get to invent how the virus works: what it does to the human body, how long it take to kill someone, how it's transmitted, what slows or stops it, and all that.

Which might be kind of fun if it didn't require so much research.

How does this virus know which passengers on a cruise ship are US officials? Pretty smart little critter!

Why not just sink the ship and drown everyone on board?

Because then it would be a totally different story; one that the author didn't feel he wanted to write. But he DID want to write about zombies.

If you feel stories should be cut right down to minimum length, why bother reading at all?
If you want to write a similar novel but using poison instead of zombies, I don't believe anyone is likely to stop you.

It would be easy to get a zombie virus onto a ship. Nowadays a drone would be used to drop a vial of the virus onto the ship, somewhere that sailors touch. But it could be put on the handrails used to get onto the ship when it's in port.
It is harder to get people to eat a poison.