Long term capital losses - I sold a boat 2017 for a $100K loss. I need this loss to offset 2018 real estate gains. Do I need to list this boat on my 2016 return?

I bought this boat 3 bdrm 2 bath boat in 2014, paid cash, so did not list as a 2nd home on 2015 & 2016 tax returns.  How do I show as a loss next year on my 2017 tax return. Do I need to revise my 2016 Tax return to show this boat?  Thanks, Marvin