I really like
Survivor of the Unseen. It's a really powerful card. If you can afford to put in some fetchlands then it becomes even better.
Medomai's Prophecy is excellent. You can keep doing the scry 2, but you can setup so that it's keeps ticking over on the 3rd Saga. So during your upkeep you go down to two counters (Chisei) naming a card, and then the 3rd Saga triggers main phase. So literally you can always name a card that you are going to cast for that turn to draw 2 cards every turn.
If your win condition is
Thassa's Oracle then
Thought Lash is an instant win with it.
Kiora Bests the Sea God is a much more powerful
Time of Ice for an additional 3 mana, so more expensive but greater upside.
With
Breath of Dreams,
Illusionary Terrain,
Tidal Control,
Quicksilver Fountain, you could have a hack card(s) to alter as needed. There are a number of them to choose from, but for example you could just play the one
Crystal Spray and it's a fairly free slot in the deck as it replaces itself.
I'm personally not convinced on the proliferate in this deck.
Adding counters doesn't seem all that great. To me it just seems like Sagas,
Blast Zone,
As Foretold,
Jace Beleren,
Treasure Map,
Umezawa's Jitte, which is definitely not enough incentive.
There is an infinite combo with
The Mirari Conjecture and repeated 2nd Saga with an extra turn card, like
Time Warp. It looks like you should add a few extra sorcery to the deck to make the
The Mirari Conjecture a better card in the first place.