Rebuilding a dock or launch extends the life of the asset. Therefore it is a Capital Improvement and you depreciate it. How long is another story altogether. Fresh water it should last probably 10 years so depreciate it over no more than 7. Salt water 5

Agree that it has to be capitalized but the recovery period is not arbitrary. I would classify it as a land improvement 15/20

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.

A dock either is floating (still attached, just indirectly) or fixed. You can read some helpful info here:

https://news.bloombergtax.com/daily-tax-report/insight-13

For how the IRS deemed a floating dock is Real Estate Asset (a link is provided there for the ruling).

Pub 946 includes this: "Land and Improvements

Land and land improvements do not qualify as section 179 property. Land improvements include swimming pools, paved parking areas, wharves, docks, bridges, and fences."