George and Lynne live by the river. We know this because they have been boating a number of times and their local pub is called the Ferryboat Inn which implies it is in the vicinity of water. Those facts mean it is not peculiar that they would be walking by the river. The only peculiar thing here is George's jumper: a garment we have never seen before.
George has undoubtedly assumed that Lynne has used the phrase 'pushed the boat out' to mean that Ted and Tina spent a lot of money on a day they could remember for a while. Perhaps it was a landmark anniversary for them. She has however meant that they did actually push a boat on to the river for their anniversary. She thought that prefixing the phrase with the word 'really' would lead George to understand that she wasn't talking figuratively. She should have used the word 'literally' to properly convey her message.
We come across a boat sinking. Are we to assume that this is Ted and Tina's boat? The water has got decidedly choppy at this part of the river. It was not the best place for a novice to go rowing. With the boat still sinking, it may mean that Ted and Tina have recently been here. Herein lies the tragedy of the walk; George and Lynne have gone to the place where Ted and Tina drowned to pay their respects. They look surprised as they didn't expect such a vivid reminder of the accident. Then again, they aren't laying down any flowers so their excursion is fuelled more by morbid intrigue than by mourning.