As a whole, I think Seaside does a much better job with card selection that the original Base cards. If I play a few dozen games with just Seaside + Base, I'm probably going to buy every Seaside card multiple times. All Seaside cards have a reasonably likely scenario where they are helpful. However, there are several Base cards that I'm probably not going to play in any of the dozens of games. All of these low play value Base cards were removed in the 2nd edition. As a whole, I think Seaside is a good expansion that really enhances the game. That said, there are some things I think could be improved.
Looking at the Seaside cards as a whole, one of the first thing that stands out to me is the large number of $2 cards and lack of any cards $6+ cards. The wiki mentions this was intentional to balance out Prosperity. Now that there are more expansions, balancing out Prosperity is probably not as important, so I'd favor having the card costs more similar to other expansions -- dropping 1 or 2 of the $2 cards, and adding a $6+ card. I also think 3 villages is high for a single expansion, particularly with the large number of cantrips, so I'd favor dropping one of the villages.
Most of the cards also have relatively straightforward implementations, leading to straightforward strategies that are often quite similar to Base. It's rare for me to stumble across a unique strategy that I had never considered, yet is very powerful, or cards that combo in special and powerful way that I had not considered. This reduces how much long term pleasure I get from the expansion by discovering new things. Compared to base, many cards play well with blunt strategies, such as buying as many as possible. For example, bluntly buying a large number of Wharfs, Ghost Ships, Light Houses, or Fishing Villages is usually going to work out a lot better than buying a lot of Smithy/Council/Library, Militia, Moat, or Villages. Along the same lines, there are fewer good, less common strategy enablers, such as Rush enablers and Gardens enablers, although Swamp Hag + Gardens can be an interesting and effective 3 Pile / Rush type game. I'd favor a larger portion of cards with unique and less obvious interactions that require more finesse to play effectively, and give more long term play value from discovering new things.
Regarding specific cards:
Pearl Diver -- I mentioned that I'd favor dropping 1 or 2 of the $2 cards. Pearl Diver is the obvious choice. While there are many times I'd buy it, it rarely leads to any particularly new or interesting strategies and is a card few would miss.
Native Village -- I mentioned that I'd like to drop 1 of the 3 villages. While I do like Native Village and it does lead to unique strategies and interactions, if I had to pick one of the 3 villages to go, this one would be my choice. It also helps reduce the number of $2 cards.
Embargo -- Embargo seems to be a common choice in the poll, however, I'd want to keep this card. Embargo can completely change the game and make games much more interesting. It's probably the card that is the least automated and requires the most thought for me to play in the entire expansion. For example, I recently had a game in which both Silver and Gold were Embargo'd early on. It caused us to play unique strategies that I'd never play otherwise and may never play again. While it's not the most effective card, it can be a game changer and adds a lot of play value.
Pirate Ship -- I like Pirate Ship. It leads to unique strategies and play style and a much greater degree of player interaction than with typical cards. Pirate Ship + ... may be the best strategy if your opponent is doing something centered on money, but may not be the best strategy if your opponent is doing something else. Its power also varies dramatically with number of players. Nevertheless, I voted for Pirate Ship. It rarely works well without some kind of good enabler combined with an opponent strategy, or with an extreme slog (Sea Hag without trashing), so it doesn't get a lot of play time. Pirate Ship games also seem to be unpopular with a lot of players. When I play a series of Seaside+Base games with a particular player, it often ends when we hit a Pirate Ship game. After hitting a PS game, they want to switch to different expansions. So I'd like to keep the general PS concept, but have it implemented differently... sort of like how Thief was improved to Bandit in the base 2nd edition.
Sea Hag -- I have mixed feelings about Sea Hag. Sea Hag is a game changing card that can lead to unique strategies. For example, I mentioned the Sea Hag / Gardens 3 pile / rush above, as well as it making Pirate Ship effective (with no trashing). This is great. However, it also can be annoying to have a long game with your deck full of garbage and not being able to do much. I once had a game Sea Hag slog game that lasted ~40 turns in which my opponent and I were mostly exchanging garbage back and forth with Ambassadors. That was the most annoying game I can recall. I guess it depends what Sea Hag combines with in the kingdom. Ideally there could be a similar replacement card that still led to unique and creative strategies, but was generally more enjoyable to play.
Navigator -- Navigator was a borderline card for me. It doesn't get a lot of play time and isn't as interesting as most cards in the expansion to me. There is minimal interaction with opponent, it's rarely a game changer, it takes time to resolve on MF (need to specify order of 5 cards in cases when don't care about order)... it's just okay and not especially memorable. I wouldn't miss it, if it was replaced.
