Having played against modern Merfolk myself I can say with certainty that Thassa is very good in this deck. The game usually goes something like this:
T1: land, Aether Vial
T2: vialed Cursecatcher, lord
T3: vialed Lord, active Thassa
The Scry 1 is extremely important for maximizing the quality of your draws. Aggro decks need to keep drawing threats to be relevant and Thassa helps with that quite a bit. My best friend pilots a fish deck which runs 2 Thassas and that seems to be a good number. I would not suggest running 4.
About Remand: My opinion is that you can run 4. I'm not saying for sure run 4, Mana Leak is a very good counter also, but if the game goes longer than 5 turns Leak becomes significantly less useful and is next to useless against Tron. Merfolk should be fast enough that if you counter just 1 spell that would mess you up that probably delayed your opponent long enough for you to swing for the win.
Side note: Put something to counter Ensnaring Bridge in your sideboard. Spell Pierce might be all you need.
2nd Side note: Twisted Image is amazing.