When the tiger left the boat he was starving! He could have turned round and eaten Pi but didn't. Pi was at his weakest and the Tiger would have known that. Pi was easy prey that the Tiger did not take That was him showing that there was a bond there.
Also Tigers are proud creatures. Turning his back on Pi is his way of saying "You don't own me, you never did, you never will". Pausing briefly before moving into the jungle does show some mark of respect however