I wouldn't call it a land improvement. It doesn't improve the land, it improves the asset. But I do agree that it's not arbitrary, nor would it (I think) depend on the salinity of the water. Since all improvements in 2019 are 15 years, I guess it doesn't matter what kind of improvement you call it, does it? 

I might make an argument to expense it, especially if the total cost is < $1k. Taking $67/year for 15 years seems petty, and I doubt the IRS would complain if you wrote that off as an expense in Yr 1, as long as it's under $1k.