If you are interested in staying in the Ashram of Amma you do not have to reserve ahead, unless she is in the center, when tens of thousands of people come to visit her and it is difficult to find a room.
The day you want to leave the Ashram, you have to go to the docks next to it, where the ferries stop. The ticket you bought in Alleppey (or Quilon) will still be valid. Get on the ferry, and show it to the crew.
In the picture you can see a local fisherman with his original sail.