Why is the sea trip rejected by the three friends in "Three Men in a Boat" by Jerome K. Jerome?

In Chapter One, Harris suggests that the three friends embark on a sea trip. J is the first one to disagree. He argues that a sea trip is a great experience if one can take a few months for it; however, if the sea trip will only last a week, it can be a devastatingly joyless experience. J maintains that it usually takes a week to overcome the propensity of getting seasick, and by the time one does, the trip will essentially be over.

J relates the story of his brother-in-law, who made the mistake of going on a short sea trip. By the time he got to Liverpool, his brother-in-law was anxious to sell his return ticket at a discount; he had had enough of the sea and wanted to take the train home. Evidently, the short sea trip had been too taxing for him, and he maintained that one could get more exercise sitting down (presumably being seasick) than "turning somersaults on dry land."

Next, J relates the story of his friend, who went on a week's voyage around the coast. This friend paid full price for a week's worth of food that he never got to eat. The initial fare was unappetizing, and then J's friend got seasick. This left him having to survive on thin captain's biscuits and soda-water for four days. By the time he was well enough to sample the food he had paid for, the voyage was over.

J tells his friends that he worries George will suffer the same fate. For his part, George maintains that J and Harris will likely be the ones to get seasick before he does. He declares that he's never gotten seasick, even during tempestuous sea trips. Then, J offers some strange advice on balancing one's body during sea trips; he argues that it is "an excellent preventive against sea-sickness."

You stand in the centre of the deck, and, as the ship heaves and pitches, you move your body about, so as to keep it always straight.  When the front of the ship rises, you lean forward, till the deck almost touches your nose; and when its back end gets up, you lean backwards.  This is all very well for an hour or two; but you can’t balance yourself for a week.

Upon hearing this terrible advice, George pipes up that they should go up the river instead. He argues that they will have "fresh air, exercise and quiet," and eventually, this is what the three friends decide to do. They reject the sea trip because none of them can agree that a week's voyage will prove enjoyable.