Loadout a soldier with a backpack. Switch to 3rd Person to verify the backpack is on the player. Embark onto a boat. The backpack will not be present on the character. Verify with 3rd Person View. Disembark boat. Backpack will reappear on the character. Verify with 3rd Person View. Repeat as many times as needed. Backpacks off while in boat. Backpacks on while out of boat.
http://i.imgur.com/g9ZZieo.jpg