Hi!
For a budget list, this is still really solid! I like the additions of
Bident of Thassa and
Cold-Eyed Selkie to make up for the card draw since the deck doesn't have Silvergil Adept.
Aquitect's Will is a card I've always thought could be interesting in a Merfolk deck, and it definitely fits the budget theme. It doesn't have the same "destroy-esque" effect as Seas's claim or
Spreading seas, but it draws you a card for 1 mana, turns on islandwalk, and is a merfolk spell for Meerow Reejery. Just something to think about
As for the whole
Mana leak vs.
Spell Pierce thing... I think what Inphy77 was getting at is that
Spell Pierce is better for countering the cards that a Merfolk deck needs to be most worried about, namely removal and sweepers like
Wrath of God /
Anger of the Gods. I know
mana leak makes them pay more, but it harder for a Merfolk deck to hold up 2 mana than it is to hold up 1 for
Spell Pierce. In the end though, a lot of it comes down to your local meta. If the Worldspire Wurm deck is popular where you play and you have a hard time beating it, then
Mana leak is going to be better for you there than
Spell Pierce (or maybe even consider
Vapor Snag).
Anyway, I hope you found this helpful, and good luck!