I have Doom (249 w/ Coordination) and Tempest (116 w/ Submerged).

If I position Doom behind Tempest (who has his Summon the Elements ability active), I can attack through Tempest, but if my opponent has a character with the Mystic Team Ability will he be able to shoot through Tempest?

Coordination says the character can't block line of fire to and from Doom (because they have the same team symbol), but Submerged says Lines of Fire to Tempest are considered blocked (because his ability is active).

So basically, the question is does my opponent's attack technically even draw line of fire to Tempest?

I'm assuming my opponent will be able to attack Doom due to the wording (and that's how I have played it), but I don't know if there's a ruling involved.

Thanks in advance for the help.

You're correct. An opposing character (wether he possesses the Mystics team ability or not, by the way) would be able to draw a line of fire to Doom passing through Tempest: since the line of fire isn't drawn to Tempest himself, Submerged doesn't come into play; and, anyway, Tempest is ignored when drawing line of fire to Doom, so whatever powers or feats he is using is irrelevant.

I understand the Line of Fire isn't drawn to Tempest himself, but since Doom is directly behind Tempest, it would pass through him, blocking the line of fire (In regular situations).

This is all under the assumption that the Line of Fire passes through the square Tempest is occupying.

Yes, I answered having that in mind.
It's the drawback of Coordination: Doom ignores Mystics characters when drawing lines of fire, but likewise, any character (friendly or opposing) drawing line of fire to Doom also ignores Mystics - no matter what powers or feats said Mystics are using (which is why Coordination only costs 6 points).
That's also why Coordination is better when assigned to characters who can use Stealth (or an equivalent effect).

I see, because Coordination says the Line of Fire isn't blocked by the character.

But then, isn't that what Stealth says (if the Line of Fire crosses Hindering Terrain, it's considered blocked)?

So is Stealth blocking the Line of Fire, or the character using Stealth blocking the Line of Fire?

Yes, but only when the line of fire is drawn to the character using Stealth.

Hindering terrain blocks the line of fire, because the character is using Stealth (assuming the line of fire is drawn to the character using Stealth).

To use examples:

A...HBC

A = Opposing character
. = clear terrain
H = Hindering terrain
B = friendly character
C = friendly character using Coordination (with the same team symbol as B)

Line of fire from A to B is clear;
line of fire from A to C is clear: B should block it, but doesn't because of Coordination.

Now let's say B can use Stealth:
line of fire from A to B is blocked, because B is using Stealth and the line of fire crosses hindering terrain;
line of fire from A to C is clear: B should block it, but doesn't because of Coordination; wether B can use Stealth or not doesn't matter, because the line of fire isn't drawn to B himself.

Now let's say B and C can use Stealth:
line of fire from A to B is blocked;
line of fire from A to C is also blocked: B doesn't block it (he's ignored thanks to Coordination), but the line of fire crosses hindering terrain, and C is using Stealth, so it's blocked.

I always assumed that the effects of feats were optional, much like powers and abilities. Because of this I've played that coordination could be turned off when it wasn't needed, much like stealth.

Am I wrong?

And why?

You're right, generally speaking. All powers, feats, and abilities are optional, unless stated otherwise.

That's why there's this entry in the Player Guide:
"COORDINATION
• This feat is not optional."