Mm, such rose-tinted propaganda. Ebb and Flow heals more than double Sona can and will still bounce for more damage than Hymn's bolts. On the other hand, its damage is higher than Hymn's bolt+on-hit and again will bounce to heal more than Sona could dream of. Not to mention Ebb has better scaling on its heal than Aria. For the use of only a single spell, Nami outclasses Sona when she casts 2 spells.
Then we have Aqua Prison, yes its a stun, but you conveniently chose to not include the fact that just like Flow it does higher damage than Hymn's bolt+on-hit. Better damage, harder cc and equivalent scaling.
Next is Tidecaller's Blessing. Each individual charge has higher on-hit damage than Hymn's and landing all 3 once again out-damages bolt+on-hit. This all before adding in the slow that each charge inflicts. Sona only gets a slow off Tempo which requires 3 spells and she has to apply herself, she can't donate it to allies while she's out-of-range. Higher damage and a slow, a better slow at that.
Finally is Tidal Wave, it has a flat 100 mana cost, nearly thrice Crescendo's range. It applies a knock-up, which everyone knows is stronger than a stun and 2-4 seconds of slow on top of that. Sure Crescendo wasn't nerfed like the rest of Sona's abilities, but Tidal Wave is just blatantly stronger regardless.
Then you have to note Nami's passive makes all 4 of her abilities MS boosts whilst Sona only has 1. There's also the fact that Nami's abilities aren't melee range like Sona's, she can safely sit in back and buff teammates and even Aqua Prison keeps her out of danger. It's amazing to think that all 3 of Nami's abilities deal damage, yet at the same time they each individually outclass Sona's 1 damage ability.
To conclude, I'll compare their base stats: Nami has +12 base HP; +40 base mana; +1 base AD; +0.1 AD/Lvl; +5 base MS; = base HP regen; = HP regen/Lvl; + 0.31 AS/Lvl; = base AS; + 0.7 armor/Lvl
That's 7 base stats that are higher on Nami and 3 that are equal. Summarized, Nami has more damage, healing, cc, mobility, range, resource management and better base stats.