How to make asset with many objects one mesh? (3d)

Im attempting to follow a tutorial to make a boat float in my ocean.
In the tutorial the script ect are applied to a boat with one mesh and cannot be applied to parent game object.

my boat has many components and when in play mode the components fall and float as individual peaces.
ship is bought from asset store.

how can i tell unity that these objects belong to one solid structure?

placed objects under hull object and applied scripts and collider to that. seems to work for now