The Booty Bay Alarm Bell can be found on the upper part of The Old Port Authority. Even after the wall-walking exploit was fixed, it is reachable by going straight to the right after running out of Booty Bay. From there you can make your way up to the top of the mountain behind Booty Bay. Mages can use Slow Fall and float down on the tower, but other classes can use the Noggenfogger Elixir or something with similar effects.
The bell doesn't have any function, the cursor turns into an action wheel when hovering it, but even when it is reached it has no effects whatsoever.