505 feet, assuming you can hit
Activating the folding boat
The rules for falling state that a character cannot cast a spell that requires a standard action unless the fall is greater than 500 feet. I assume that is because items in Pathfinder fall 500 feet in 6 seconds.
If we give a somewhat generous assumption that the flying character can hold the boat while it is in its smaller deployed state it will take one standard action to activate the boat to get it into its largest state. Therefore the flier would have to be 505 feet above the target, or 1005 if they cannot hold it while the boat has been activated once.
Normally ranged sneak attacks can only be performed within 30 feet of the target, but Sniper Goggles circumvent that restriction. A 9th-level character might be able to afford a pair of those. On the other hand, an attack roll is still required, and according to the falling damage rules above dropping an item on an enemy generally have a range increment of 20. Thrown weapons can normally be used up to 5x their range increment, although I might rule that dropping an item can be used up to 10x the increment, which would get to you 200 feet. Using a large item such as the boat in question may increase the range increment to 50, which still would not be enough to be able to hit someone 505 feet below you. If you could find some way of circumventing the range increment restriction or increasing the range increment itself you would be able to take the shot but would still have around about a -20 penalty on your attack against their flat-footed touch AC (which is probably 10) so you had better roll at least a 20 because 20 + 6 (BAB) + 4? (other bonuses) - 20 = 10, unless you can get better bonuses than just your dexterity. You might be given a bonus for using a large weapon that, if it gets in the right square, is essentially guaranteed to squish the target.
There are rules for the amount of damage a dropped item deals to someone it lands on (see Falling Objects, under Falling). An object does double damage when dropped from over 150 feet, but as seen above that is not the limiting factor in the height requirement.
Large objects deal 4d6 damage, although there is probably a penalty due to the hollow nature of the boat, but a bonus from being over 150 feet high. The boat might do about 6d6 points of damage instead in these circumstances. Additionally, sneak attack damage would help kill the target. A 9th-level rogue would get +5d6 sneak attack. 6d6 + 5d6 averages at 38.5 damage, giving you a good chance of killing a 30hp bandit in one shot.