To help other players with their boat order, simply visit the farm of the person you would like to help. If they have requested help to fill in boat orders, you will see a yellow exclamation mark hovering near the boat. Tap on it and you’ll be able see the orders that need to be filled and those that have already been taken care of. When there is no exclamation mark, it either means the player doesn’t want or need help.
Tap on the empty box to fill an order. This will show you how many items are needed and how many you have in store. You can only close the box if you have enough of that specific item available; once completed, you will get rewarded (experience and coins).
You can also ask other players to help you complete your own boat orders. You can do this by tapping on your boat and then pressing the "ask for help" button on the boxes you need help with.