Can Hypnotize replace Ice shot?

lol, no.

1. It targets one monster (as mentioned)
2. It has a minimum range
3. It can only be active on one monster at a time
4. Monsters don't do much damage to each other
5. Waste of important SP

EDIT: And what Takebacker ninja'd me with, too.

You forgot the fact that you can't even use it while on the ship.