What reason did each friend put forth for his being against a sea –trip ? what was the actual reason?

Harris brought up the sea trip, and thus, he cannot be included.

Jerome:

I objected to the sea trip strongly. A sea trip does you good when you are going to have a couple of months of it, but, for a week, it is wicked.

Jerome also cited that he'd be worried about George.... who responded that he'd be fine and would enjoy a sea trip.

So I set my face against the sea trip. Not, as I explained, upon my own account. I was never queer. But I was afraid for George. George said he should be all right, and would rather like it, but he would advise Harris and me not to think of it, as he felt sure we should both be ill.

The actual reason everyone is against a sea trip is the worry of sea sickness. Trips aren't fun if they're spent in your room and cannot eat.