Testing to see how much cards opponent plays in the first 7 turns. (blanks are because I got lazy or forgot)
1: 3 3 4 4 4 2 1
2: 0 1 1 2 1 1 2
3: 2 1 1 1 2 1 1
4: 1 2 1 2 1 1 1
5: 2 2 1 1 1 1 1
6: 1 1 1 0 2 1 1
7: 1 0 1 2 0
Averages are 1.43, 1.43, 1.5, 1.57, 1, and 1. So the total average is 1.32. So averagely a cymothoa is a 3.96, roughly 4 momentum'd attacker.
BUT
It is absolutely impossible to get out cymothoa in the first turn unupped. It's possible to get it out second turn, but impossible to get it out first turn (unless you get 2 novas, 2 photons, 2 immolations, and a cymothoa), so the first turn should be discarded.
New averages are 1.17, 1.17, 1, 1, 1.5, 1, and 1. New total average is 1.12. Averagely, a cymothoa does 3.36 momentum'd damage in the first 7 turns of the game, making it slightly better than blue crawler, but not as good as toadfish.
With 4 water towers or upped water pendulums this can come out first turn. Mix and match with 1 or 2 novas. 4 Pillars / pendulums first turn is a little on the high side, but not that uncommon.
Alternately, if you win the coin flip, silence can work as a stall in an upped deck as well... then you stall them from playing cards till these can come out.
Still, I dont think its that highly OP with fractal...
Is the damage considered spell damage btw? If so, then mirror / jade shield is a viable counter... I actually think these shields tend to be underplayed and could use a little more love so that would be a good think in my opinion.