From what I have played, building a wall is pretty pointless, is this supposed to be the case? The shark can still attack a raft piece even if there is a wall in the way, and I haven't noticed any change in how the shark attacks or how long it takes.
I have a suggestion about it: if the shark attacks a raft piece that has a wall in the way, then the wall will be destroyed, but the raft piece and anything on it will be safe. This would provide a reason for building walls and would add a bit more defense against the shark.