There are a few ways to get a boxing glove in a boat for treasure. The most common way is to find a chest that contains one. These can be found in many different locations, including shipwrecks, buried pirate treasure, and even some merchant stalls.
Another way to get a boxing glove in a boat for treasure is to purchase one from a blacksmith or other craftsman. This is generally more expensive than finding one in a chest, but it guarantees that you will get a usable glove.
Finally, it is possible to loot boxing gloves from the bodies of defeated opponents. This is not always reliable, as some opponents may not be carrying any gloves, but it is worth checking if you are desperate for one.