The pentagram was used to keep demons out. As Avery believes the demon is taking people, I don't think it needs any explaining.

Is it me, or was that the most stupid rescue plan ever?
Everyone else appears to have died, Rory is drowning, let's all go 'poof' with the siren and that will sort everything out?

the pentagram NEEDS explaining.

any yes it was a giant LEAP OF FAITH

isnt a pentagram, surrounded by a circle, surrounded by a triangle believed to keep demons out? (known as THE DEVILS TRAP).

the pentagram has links to the bible ,ancient greece and omega --also the minotaur in episode 11
also on the badge on rivers camoflauge uniform as far back as Forest of the dead ..ans throughout this series both screened episodes and futre episodes

Greek myths seem to be a theme -- the symbols . the minataur the labrynth (hotel in ep 11 ... )
and this episode is loosely based on the Odyssey

it must be noted the crest at the bottom of the badge /logo uses a 5 point star
again in greek legend refering not to evil but

Pentagrams were used symbolically in ancient Greece and Babylonia, and are used akin to the use of the cross by Christians and the Star of David by Jews.
and The term Alpha and Omega comes from the phrase "I am the alpha and the omega" (Koiné Greek: τὸ Α καὶ τὸ Ω) in the Book of Revelation (verses 1:8, 21:6, and 22:13).

coincidently also used in the US air force badge (which this does resemble)
hope that explains it

I didn't like very much the episode. Plot derived from The empty child/The Doctor dances, a parallel universe very badly explained when it is said in season 2 that since the end of the TimeLords it is now nearly impossible to travel in parallel universes. And then the stupid thing "Oh, were all alone, lets suicide with the mermaid to see what it does !" Because there were actually no traces of a potential intelligence on her behalf.
So, I'm not convinced this week...

What happened to the pirate the kid cut? He was still in the gun room with Amy and Rory, piling barrels in front of the door, then when the captain and Doctor return (after the other pirate was taken) he's not there and never mentioned again. Did they cut a scene where he was taken? It really bugged me when watching it.

The parallel universe doesn't bother me. Martha in the Mirror did it in the middle of series 3, the entire multiverse has been rebooted, and there's more than one kind of parallel universe (Everett superpositions, the brane next door, the other side of a black hole, ...).

But the rescue plan... I have to watch it again to see if the Doctor had any good reason to believe it was going to work, or if it was another hail mary play like in Amy's Choice (and not even a 50/50 chance this time). That's the one thing that bothers me about this episode. Everything else, I loved. The atmosphere, Hugh Bonneville, Amy the pirate queen....

Did I miss the part when the Siren took the Boatswain?

I remember Toby cut his hand, then the other pirate grabbed the keys and ran out, shot at the Doctor and Avery then was taken by the Siren himself. But back in the powder room the Boatswain's gone and doesn't return until appearing on the spaceship at the end. So did it go by in a flash or is that a plot hole?

Did I miss the part where this episode was good?

I have to say, Steven Thompson was also the writer of the weakest episode of Sherlock series 1. I tried to give him the benefit of the doubt, but this didn't help my impression.

As I remarked in a previous post, 2 out of 3 for series 6 is not bad at all. Hopefully episode 4 will put series 6 back on track again.

It's not that I don't like Stephen Moffat's writing, I do (my favorite episode last series was the two-part Weeping Angels episode), yet I found late in series 5 and during this episode that I really miss Russell T. Davis' writing or at least his supervision and editing of the writing.

Watching it again:

The Doctor's sudden "let the Siren take us" plan still seems stupid. Also, they should have given us some hint that it took at least a few weeks for Avery and friends to learn to pilot the starship, and for the boy to recover from typhoid fever. There are a lot of smaller plot holes, too. More importantly, there's nothing too original or thought-provoking in the story, and no interesting commentary on any of the TARDIS crew's characters.

So, overall, the writing wasn't terrible, but it's definitely the worst so far in the Moffat era.

That said, I still enjoyed it. The acting was great (especially Hugh Bonneville and Karen Gillan), the atmosphere was perfect, the directing was solid, the action set pieces were exciting, the funny moments were funny, etc. I'm glad I watched it, and it's still better than most of what's on TV on either side of the Atlantic.

I thought the sonic screwdriver didn't work on simple locks - bah

No, it doesn't work on wood. But here it worked on the metal piece of the lock.

Just watched it again, still rubbish...First, they just let people go to the mermaid without doing anything, except Amy who protect Rory. Then the pirate wounded by the boy just vanish, lets suppose he get outside and was taken because he is seen on the deck at the end. After that, the decision to suddenly go with the mermaid based on no observations, not like in Amy's choice where there was actually few clues. And then the rubbish kiss of life scene, when all they had to do was fetch a medical team with the Tardis and bring the on the ship to save everyone. Other complaint are the parallel universe which lock more like a time rift to me, the complete absurdity of the nurse who bring every injured men in the sickbay (just imagine if the captain, while doing a very dangerous manoeuvre on the edge of a black hole, cut his finger...) and the ending with only the five pirates actually seen in the episode on the bridge when they could have put the ones from the sick bay too.

And another thing. They didn't had to throw all the treasures. The mermaid comes from the reflections. No light, no reflections, they just had to lock them up in a dark room.
Rubbish plot is rubbish...

A holo nurse who can only come out of reflections?
Bad design.
Smash the mirror, now you have millions of places to come out of.

Sea 'still enough to be reflective'? On what planet?

Big red snarl when 'sterilising' things? Why?

Siren pulls everyone through to the other reality.
Who sends them BACK?

The only plus points are that if not for poorer episodes, how can you compare the good ones?
And it was beautifully shot.

This one is not a reflection of Moffats writing, although it does reflect on the difference in style he has as 'showrunner'
More hands-off than RTD and tighter deadlines.