Marine Salvager: Action: Exhaust Marine Salvager and pay X to choose a support card in any player's discard pile. Then, put that support card into play under your control. X is equal to the printed cost of the support card.
If I activate him to pick a negative Attachment (such as Indebted from my previous rules question), can I still put that attachment on one of my opponent's characters (I'm just a little unsure what "under your control" means)?
Yes, the attachment still must meet all play restrictions (that part in the text box that says where to attach it). The "under your control" part is merely to clarify that if you get a support card from your opponent's discard that it will be under your control, not his.
"Under your control" is unrelated to what card the attachment is attached to. It just means that you are the one who controls that card.
For instance, if the card had an action on it, then you're the one who could activate that action. It could also matter if some other card says "choose an attachment you control" or "a support you control" or your opponent has one that says "an X controlled by an opponent".
By itself control doesn't have a lot of meaning on an attachment, but it has implications that could matter when other cards or abilities on the card are taken into account.