Ok, so the boat goes over a wave and accelerates upwards, the tension in the string decreases. Whoop-dee-doo. Your little scenario isn't any different to mine except that you can measure the acceleration which isn't required. Whatever example people choose, just make sure you can destinguish between non-inertial, inertial and between stationary and constant velocity.
Oh while you dishing out advice, I might dish out some of my own. Make sure you relate that to a specific example otherwise you'll get marked down. Whether that example involves a boat, plane, car, bus whatever, you still need an example.