I like Treasure Map a lot as is and don't think you should be able to open with two. The main thing I would want to change is make Sea Hag put a Curse on your Deck without discarding the top card. On average that would make the card ever so slightly stronger but would eliminate those games that you feel like you lost just because you got your Sea Hag or your Chapel hagged.
Pearl Diver is weak and boring so it could be replaced with a real card but Seaside doesn't have the problem that Base and Intrigue had where on most boards you just don't get to do anything. One dud doesn't hurt the set too much.
Merchant Ship is boring and not the greatest card, I doubt many people would miss it.
Embargo is really cool and I wish it was good more often. It could get "When you buy this/gain this during your turn, discard your deck." That would make it much faster and more unique. When you open 5/2 with Embargo and a good 5, which is the case where you don't want Embargo buffed imo, it doesn't even make a difference (and with a 2/5 opening it actually gets worse). Since you're only playing it once anyways it could get +1 action (instead of the other change, probably not in addition to it but maybe).
Duration Throne is a great idea, could be as simple as
Captain's Cabin
4$ Action - Duration
You may play an action card from your hand. If you do, play it again at the start of your next turn.
//edit: I immediately noticed that Captain's Cabin probably shouldn't work on Durations.