Bobbing About! is a magazine story.
A big pleasure boat called the S.S. Mermaid has been moored by the pier. Thomas has brought Stephen, Bridget and Lady Hatt to the seaside. The children see the big ship and ask their grandmother if they can go on a boat trip. Lady Hatt agrees and they join the queue of passengers waiting to board the boat.
When everyone is aboard, the ship departs and heads towards the island. Stephen is very pleased to see a seal which has come to say hello. It is a long trip and, further out to sea, the waves grow bigger. The boat starts to bob about and Lady Hatt reassures the children that it is perfectly normal and safe. However, Stephen and Bridget feel very strange whenever the boat rolls up and down. The children enjoy seeing the seals, but are pleased to reach dry land again.
When they finally step ashore, it seems to go up and down for a moment, too. Both Stephen and Bridget feel very dizzy indeed. The children spend the rest of the day on the beach, paddling, building sandcastles and playing with a beach ball.
At last, Lady Hatt tells them it is time to go. Just then, Thomas returns pulling the tea-time passenger train. At the seaside station, Stephen and Bridget tell Thomas' driver all about the boat trip. Thomas overhears and Stephen is glad that engines don't bob about like boats.
On the way back, there is a broken rail. Thomas has to go around it on an old loopline that is hardly ever used. Thomas has to go very slowly, but the track is very bouncy due to the wooden sleepers becoming loose. Thomas tries to travel smoothly to please his passengers, but however hard he tries, he cannot help rocking to and fro. Annie and Clarabel sway from side-to-side too. Stephen, Bridget and Lady Hatt all bounce up and down on Annie's springy seats.
At last, Thomas pulls into the main station where the Fat Controller is waiting to apologise to the passengers for their late arrival. Stephen and Bridget step onto the platform, feeling dizzy once again. Bridget looks on the bright side; thanks to Thomas, that train trip was worse than being at sea.