Nah, just make them an item of greater disguise and forget about it. If Doran's so paranoid he can come up with something else.
Uh, I don't see why this is needed for King's landing, unless the underwater bit is part of your Deep One's alpha strike thingy?
There is one point that I think you might have missed that's been brought up in the past- If Viserys only prepares for generalities, and does not have a specific plan, then divinations are obstructed, a tiny bit. Maybe not against gods, but as seen with Bloodraven, a really powerful mage, regarding Waymar's sister's situation, if the subject of the Divination does not know what they are going to do, then the Divination does not tell the caster what they are going to do.