Crossroads - If I'm in a classic rock mood, "Crossroads" by Cream. If even more classic than that, the original Robert Johnson version. Or, if I'm feeling hip-hoppish, I'll think "Tha Crossroads" by Bone Thugs-n-Harmony.
Bridge - Usually "Under the Bridge" by Red Hot Chili Peppers. If in a more depressed mood (i.e. ten-game losing streak), "Bridge Over Troubled Water" by Simon & Garfunkel.
Wishing Well - "Wishing Well" by Terence Trent D'arby is my usual choice, but sometimes I'll start singing "Goodnight Girl" by Wet Wet Wet instead.
Watchtower - "All Along the Watchtower," of course. Only the Jimi Hendrix version, though.
Island - ..."La Isla Bonita" by Madonna. Yeah.
Navigator - Reminds me of Nami, so I start whistling whatever "One Piece" theme pops into my head.