While these cards are both solid, they suffer most from not being able to operate at instant speed.
Monastery Siege digs you deeper, but competes with the slots for
Rhystic Study and
Mystic Remora, both of which will draw more cards in a shorter amount of time. It also has almost no immediate impact on the board and requires that you tap mana on your turn for a mediocre effect.
Thassa, God of the Sea is extremely slow and, while difficult to remove, doesn't really affect the board in a significant way. Yes, scrying at your upkeep is nice as a passive ability, but it would be infinitely more playable if it cost :1: and was an artifact. Her second ability does almost nothing for us outside of corner-case, Plan F win condition strategies of making
Cromat unblockable.
What you are looking for is a bomb to dig deep into your deck to find answers.
Dig Through Time is phenomenal.
Future Sight is the high water mark. I was playing
Treasure Cruise for a while, but it's not an instant and only (I know, only...) digs three deep so I'm trying out
Stroke of Genius again because I often found myself with mana leftover right before my turn came back around. Plus, it gets quite absurd late game and actually has the possibility of being a win-condition in slow, grindy, four hour games.