In Three Men in a Boat by Jerome K. Jerome, why do the three friends think that they are unwell?

The three friends think that they are unwell because they appear to be exhibiting certain symptoms of malaise. For example, Harris and George both claim to be experiencing "extraordinary fits of giddiness," which leaves them at a great disadvantage. Meanwhile, Jerome is convinced that his liver is out of order.
Eventually, the three friends decide that they are unwell because they are overworked. Harris suggests that a vacation of some sort is in order, while George pipes up that a change of scenery is what they need. He maintains that such a change will "restore the mental equilibrium."
Jerome agrees with George, and he suggests that they should "seek out some retired and old-world spot, far from the madding crowd" to go to for a week. However, Harris wants to have a "sea trip." For his part, Jerome doesn't like the idea at all, due to the fact that his brother-in-law and friend had bad experiences on their respective sea trips.
After much arguing about the possibility of one or all of them getting seasick on a sea trip, the three friends agree to go up the river for their vacation.