Basically, the money is a vessel-person being held on bail in an in rem proceeding. To get the money out of bail you might have to pay at least two times the amount of the plaintiff's claim (per Rules of Civil Procedure). Can be a lucrative hobby defending and collecting in rem cases if you know how to pull it off.
Related:
Federal Rules of Civil Procedure (As amended to December 1, 2014) 1, Title XIII. Supplemental Rules for Admiralty or Maritime Claims and Asset Forfeiture Actions (Rules C, E and G at the least).