Does a summoned Shark get shuffled into a random open deck? We couldn't figure this one out. Shark Island says you summon a Hammerhead Shark at the start of your turn if you're there, and the Adventure/Scenario card says any defeated Hammerhead Shark gets shuffled under another deck. It seems like they should get shuffled, but we're not sure.
No. Summoned cards go back in the box unless the summoning card tells you otherwise. The summoning card is the location, and it doesn't tell you otherwise. So the summoned sharks go back in the box.