To get the secret ending, just put both guys on the moon and boat on top of them. The boat is supposed to sink with the moon, but it doesn't work always (because it was originally a bug, but creator made it secret ending for fun). You will get the ending anyway even if the boat doesn't sink. ^^
 "UPGRADE COMPLETE!"){kind=icon}
 "I Remain"){kind=icon}
 "I wish I were the Moon"){kind=icon}
 "Creeper World Training Sim"){kind=icon}