"And when my brethren saw that I was about to build a ship, they began to murmur against me, saying: Our brother is a fool, for he thinketh that he can build a ship; yea, and he also thinketh that he can cross these great waters."
There was a very specific reason Laman and Lemuel were wary of building a boat.
Before he was a prophet, when his first four sons were still only boys, Lehi had claimed divine inspiration for the first time. He had been doing well trading along the camel routes from Arabia to Egypt--and then one day he came home and announced that God wanted him to build a ship and break into the lucrative trade in cedar from Lebanon.
Lehi threw his energy into the project with a religious intensity, and the boys got excited like they'd never been before. But getting the right permits and finding the right workers was hard; expenses built up fast and Lehi had to take out big loans; the ship almost didn't get built and almost as soon as the ship was built, there was a market collapse in Egypt and not much room for Lehi in the cedar trade anymore; finally, after a bad storm in the harbor, the ship sank.
It was the family's worst disaster. Lehi had to work extra hard and take long trade trips during Laman and Lemuel's early teenage years to bring the family fortunes back again.
So when Nephi told them God wanted another boat, Laman and Lemuel mocked him. To them, memories of the first boat meant that God didn't really get involved in nautical issues.
In Nephi's memory, though, the first boat was associated positively with his father's early faith. And it set a precedent for God's later command.
The difference between the brothers was in how they interpreted their early life experience.
(A variation on this story is also told in which the first shipbuilder is not Lehi, but Joseph Smith, while the brothers are not Laman, Lemuel, Nephi, and Sam but Warren Parrish, David Whitmer, Brigham Young, and Wilford Woodruff. In that version, the ship is not a ship, but rather the Kirtland Bank.)