I think Lighthouse was supposed to be Seaside's "Reaction". Still, I like this. It also gives Seaside a Throne variant, which is neat. To reduce the confusion, I'd maybe replace "costing less than this" with "non-Skiff"?
I might tweak the Reaction part a bit, though. The wordiness is a problem, but it also doesn't help much against Sea Hag... and doesn't help at all against Ghost Ship!
Cheap Actions are a thing in Seaside, so I thought I'd tie that in. Thanks for making the interactions with the attacks clear, though, I've changed the reaction to discard a card rather than put one back. It just lessens the impact of the two attacks so Lighthouse is still useful, as well as having other uses like turning Pearl Diver into Fugitive.
You smartly prevented a mono-card-strategy but I still wonder whether this is too strong/cheap. Beyond the sifting and the Reaction this is a double Peddler and unlike Herald and Vassal it virtually never misses if you manage to put enough <$5 Actions into your deck.
Yeah, with the reaction adding a bit of value as well, and the ability to play an Action not from hand whilst knowing a bit about the top of your deck as well, it's a strong package. Knocked the +$ down to 1.
So the change looks like this:
Is this putting everything from the mat into your hand at once, or are you choosing any you like from it? If the former, there's no point in the mat at all, just set the cards aside like Haven.