We can get stuck in our minds and lean too much on prior skills instead of fully assessing the problem at hand. More likely than not, given he was a contractor, building a trench and walls seemed like a simple solution to something he has probably dealt with many time and didn't think twice about it.

I just don't understand how it could occur to no one in a group of three or four people for a whole hour. Like, even if the barrier had been fully built and worked perfectly even after considering the tide, they would have been making camp on fully soaked ground. Just what everyone wants first thing in the morning: to trudge through mud.