BB-they don't "take" the boat in that they move it anywhere. The Marshall will literally chain it to the dock where it sits, and put a copy of the order on the boat, and post notices on windows etc, saying it is in the possession of the Marshall's service. Once you have the order, you have 30 days to respond in court to the complaint. You also can respond and ask fro an expedited hearing if you are challenging the lien. If you don't respond in time, the complainant will go to court and move for an order to have the Marshall sell the boat to pay off the lien. Then they may move the boat.
In real life, most of these things get settled pretty quickly by negotiation between the parties. Once the parties reach agreement, both parties move the court to cancel the arrest order and the boat is free.