The Shark lagoon location is the last one in the SoB rulebook. No boss on it. Only four sharks, patroling the waters near the shore.

My doubts on this location concern the shape of the river that ends in the whirpool: how can it flow among the pits?

Of course, we're talking about a fantasy world, and everything is possible....but why they designed the island that way? Why those orrible pits?

Other issue: the whirpool marker is the same of the Skull Cavern entrance. But, actually, this is not a real matter: I use the "pool marker" of AoD, that it's quite similar.

In any case....what are your feeling about this island? Is it playable and funny? Is it broken? And, if yes, how to fix it?

I don't have the map handy so can't say if it's playable or fun, but my stock answer to questions like "how does X make sense" is that Descent is a board game not a reality simulator and that Descent + Logic = Madness.