I just went through this, the ex GF bought the boat.
We broke-up and it was civil.
Somehow all the money and time I spent on upgrades or slip and boat payments amounts to exactly nada.
OTOH, she spent a lot of time and energy on my house as well, and I kept that so I guess it all worked out.
Not to mention the fact that I was actually kinda emotionally attached to the boat.
It's definitely better to define very clearly up front who-pays for what.