Wow, Night's Watch players are going to hate this.

Stark perhaps more so, standing to lose Winterfell in the challenge phase or Harrenhall in marshalling.

who didn't give this card a five? 

Nightmares on the Arbor. Big gold swing for a turn right there.

Nightmares blanks gold modifiers already, you don't need to steal the card too.

Nightmare is enough to do what you want though. No need to steal a blank location.

You can steal a gold from Lannisport Treasury with it But obvious target would be Ghaston Grey... other than that there some locations which could be slight pain to loose temporary but not that many. I think for GJ player this is more benefit because of its trait Warship. And the threat of stealing is just bonus.

If you marshal first and you steal Harrenhal (Stark), you not only allow yourself to freely marshal, but also completely prevent your opponent from marshaling a strong character because you will of course sacrifice it to kill that character. Its even bigger than Ghaston imho, where Martel player will simply play defensively that turn, but will develop the board normally. 

It's a more-restrictive NM that you can set up/use reducers on in a normal greyjoy deck. However, having the ability to steal your opponent's draw or power gain for a round against passive decks opens up a slower, more meticulous and controlly greyjoy style. Even against lanni kraken just stealing an iron mines on a first snow or valar round can be crucial, or just annoying.

If you marshal first and you steal Harrenhal (Stark), you not only allow yourself to freely marshal, but also completely prevent your opponent from marshaling a strong character because you will of course sacrifice it to kill that character. Its even bigger than Ghaston imho, where Martel player will simply play defensively that turn, but will develop the board normally. 

Since this is just an action this can be used mid challenge on Ghaston so as long as this sits on the board across from Ghaston the Martel player has to play around it just as much as if you yourself had Ghaston out as well.  

So excited to use this in melee!

There are a couple of different circumstances to look at:

1. Opponent has Sea B!tch and another non-limited location that you don't have. The other location clearly prevents this rule from being a problem: "If an ability requires the choosing of a target (or targets), and there is no valid target (or not enough valid targets), then the ability cannot be initiated. This initiation check is made at the same time the ability’s play restrictions are checked" (RRGp20). So you discard your Sea B!tch which then makes your opponent's copy a valid target in time thanks to this rule: "At the time targets are chosen, any currently valid targets are eligible to be chosen. (This choice is not restricted only to targets that were present during the initiation check)" (RRGp20).

2. Opponent's only possible target is Sea B!tch. This is less clear. The FAQ says: "If a player controls a unique card (regardless of ownership) that player cannot take/gain control of other copies of that card." Maybe you can take it in this situation, too, since you aren't actually trying to take it until you can, and since nothing can prevent you from sacrificing your copy to pay the cost (and nothing can save it).

Whether or not you would be able to use their copy's ability I think isn't very clear. "An opponent’s game elements may not be used to pay a cost" (RRGp5). If you took it, it would still be under their ownership, and the word "opponent's" is possessive, which is ownership. But maybe the game just means control and not ownership for this rule.

I would think the sacrifice would occur before you take control of your opponent's Sea *****, so you aren't controlling two copies at once.

Of course, this game isn't always as simple as I'd like it to be.

The sacrificing is before taking control. That is part of why it could be done in the first situation I looked at above.

The potential problem for the second situation is that there has to be a valid target before the sacrifice (as an initiation check), even though the target isn't actually chosen until after the sacrifice. And "a card is not an eligible target for an ability if the resolution of that ability's effect could not affect the target at all" (RRGp21). So is the resolution considered under the state of things before the sacrifice (when the initiation check is made), or after the sacrifice (when the resolution would actually happen)?

http://thronesdb.com...ating_Abilities

Oh wow, a complete reversal on one of the rules in the Rules Reference. Well that changes the case where the opponent has another valid location to target.

So it looks like you would never be able to use it on another copy of Sea B!tch.