Can the anti-whale tile be used to defend against the giant squid?
Yes, you can use the anti-whale tiles to defend against the giant squid.

Although I was unable to find a FAQ for the game on Stronghold-Games website, Kevin Nesbitt who is a credited designer of one of Survives other mini expansions said the following:


  The Giant Squid and Whale are meant to be largely interchangeable, at least in how they are brought into and out of play. Since a player must select either a whale OR a Giant Squid, they may also choose to harpoon either by play of the defensive whale tile (but only one "harpooning" per tile, please!)