der Kahn
punt
A small, flat boat without a keel, often moved by punting (pushing with a long pole).
Im Spreewald haben wir eine romantische Fahrt mit einem Kahn gemacht.
In the Spreewald, we took a romantic ride in a punt.

punt / old tub
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punt
A small, flat boat without a keel, often moved by punting (pushing with a long pole).
Im Spreewald haben wir eine romantische Fahrt mit einem Kahn gemacht.
In the Spreewald, we took a romantic ride in a punt.

In the meaning of an old vehicle, 'Kahn' (old tub) is colloquial and slightly derogatory. It describes a vehicle that someone considers old, slow, or unreliable.
Mit diesem alten Kahn von Auto will ich nicht in den Urlaub fahren.
I don't want to go on vacation in this old tub of a car.
The meaning is easy to recognize from context. If people are talking about water, rivers, or a boat trip, it means the boat. In connection with negative adjectives like 'alt' (old) or 'rostig' (rusty) for a vehicle, the colloquial meaning is meant.
Auf dem Fluss trieb ein kleiner Kahn.
A small punt was drifting on the river.
Common activities are 'eine Kahnfahrt machen' (to take a punt ride) or 'mit dem Kahn fahren' (to travel/go by punt). A very typical verb is 'staken' (to punt), which means to push the punt forward with a long pole.
Im Spreewald kann man sich in einem Kahn durch die Kanäle staken lassen.
In the Spreewald, you can have someone punt you through the canals in a punt.
ship
'das Schiff' and 'der Kahn' can both mean a vehicle on the water. 'Schiff' is the neutral word, especially for larger watercraft on the sea, on rivers, or on lakes. 'Kahn' is more likely to mean a small, flat boat, or colloquially an old, sluggish ship. Colloquially, both can also be used for cars, but 'Kahn' often sounds derogatory and old-fashioned in that case, while 'ein dickes Schiff' (a big ride) usually means a very large or flashy car. The words can be confused because both have to do with vehicles. A safe rule is: for a normal or large watercraft, use 'Schiff'; 'Kahn' fits better for small boats or jokingly for old vehicles.

Mit dem alten Kahn fahren wir langsam über den ruhigen See.
We are slowly crossing the calm lake in the old boat.
Am Hafen wartet ein großes Schiff auf die Passagiere.
A large ship is waiting for the passengers at the harbor.