舟 Zhōu is a picture of a small boat:
You can still see the oar and a couple of people sitting in it.
It's not much used these days but it does appear in: 木舟 mùzhòu 'wooden boat' and many other characters.
船 Chuán is the modern common word for 'ship, boat':
艇 Tǐng is a small boat.
Common words: 潜艇 qiántǐng 'submarine', 舰艇 jiàntǐng 'war ship'