In the TV show NCIS, Agent Gibbs is often seen working on a boat in his basement. While it is never explicitly stated why he is doing this, there are a few possible reasons.
One possibility is that he is simply building the boat for fun. He may enjoy the challenge of putting it together and sailing it once it's finished. Alternatively, he could be building the boat as a way to relax and take his mind off of work. Working on a complex project like a boat can be therapeutic and help him clear his head after a long day at the office.
Another possibility is that Gibbs has always wanted to build a boat but never had the time or money to do so until now. He may have dreamed of sailing around the world or taking long fishing trips with friends and finally decided to make that dream a reality by building his own vessel.
Whatever the reason, it's clear that Agent Gibbs enjoys working on his boat and takes pride in seeing it come together piece by piece. It's just one of the many hobbies and interests that makes him such an interesting character on NCIS.