In what sense do they take the men?
Rashi: Prepare them to be ready for what I will command them when you cross.
Malbim: They will be witnesses to the miracle. 1
Alshich: Only those close to the Aron will see that the miracle (the water split, and began to rise) began when the feet (of those carrying the Aron) rest (touch the water).
What do we learn from "Ish Echad la'Shevet"?
Malbim: They will be among the Nesi'im of the Shevet, so it is as if as if the entire Shevet is there.