Iran's "Bavar 2" stealth plane/boat-thing is billed as a stealth craft that can cruise through the water or fly, and it's equipped with a machine gun and possibly missiles. Are you worried yet? The craft's been ridiculed before for being impractical, yet maybe we're writing it off too soon.
As for its criticisms, the Bavar 2 has been accused of being very "kit," as in it looks like a watercraft you could have ordered online and assembled yourself. Just do a Google image search for "hoverwing" and you'll see many craft like it.
There's also the big question of just how stealthy it is. Iranian officials claims that its hull makes it undetectable by radar, though videos of the thing flying show that it's pretty damn loud. The blue paint job probably works well enough — the Bavar 2 doesn't fly high enough for its red underbelly to ever really factor in — it really mostly skims above the water. Still, it looks like something out of an old sci-fi cartoon, or like the Bavar 2 belongs in a water park.
So, how could it be dangerous? Well, it really couldn't, say, fly around on a spy mission or engage other aircraft. What it would do well as, though, is as a light naval harasser. Its ability to fly makes it faster than most boats (supposedly it can reach speeds of up to 100 knots above the waves), and it's probably easier to pilot than an aircraft meaning you don't need to staff them with precious, trained operators. Then again, it probably wouldn't want to take on a warship, or really any armed boat. So who knows?