WHY WAS THE NARRATOR RELUCTANT TO GO FOR A SEA VOYAGE? WHAT WAS FINALLY DECIDED BETWEEN THEM?

The narrator was reluctant to go on a sea trip because of "sea sickness".

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.

They finally agreed upon a boat trip up the river. 

“Let’s go up the river.”

He said we should have fresh air, exercise and quiet; the constant change of scene would occupy our minds (including what there was of Harris’s); and the hard work would give us a good appetite, and make us sleep well.