Detex ProxiPen systems have an unlimited number of checkpoints. They can be assigned to a container and attached to it. The crew can scan the box as it comes on board, then scan it again as it leaves. You can sort the reports to make sure it was in-fact loaded, and also that it was actually delivered.
Detex ProxiPen TAGS are weatherproof. Salt water, rain, heat, wind.. nothing affects them. ProxiPen readers are small, fit in a pocket, take a lot of abuse, and are also unaffected by salt water.