What great loss does Pi many times lament in chapter 83 of Life of Pi?
Pi Patel laments the loss of his raft many times in chapter 83 of Life of Pi.
Although I question whether your request was meant for the whole book or a single chapter, there is no doubt that Pi Patel uses the word "loss" to describe the devastation of losing his raft in this particular chapter.
I noticed the loss of the raft at dawn. All that was left of it were two tied oars and the life jacket between them. ... The loss of the raft was perhaps not fatal to my body, but it felt fatal to my spirits.
The reason for the loss of the raft was the ravaging storm of the chapter. Very different from Pi's experience with water in chapter, 82 where Pi is able to harness the water in order to survive. These two chapters together are possibly meant to show Pi's control (or lack thereof) of water. In chapter 82, Pi is able to control water just as he can control the animals in a zoo. In chapter 83, Pi is insignificant next to the great power of the water in the storm. In this chapter Pi can NOT control it and is left without his raft.
If you take a look at eNotes themes, you will see that he loss of the raft connects very nicely to the theme of "As above, so below: what is true for one arena of life is true in others."