All attacks with Tsunami actually does benefit from ring of tides.
Both the effect dice and the amount of red dice rolled.
The reason for this is the wording on ring of tides.
Unlike most other effects it does not grant ranged +x.
The rule for zone attacks is that they do no benefit from ranged +x. Ring of tides is different.
A side note: a creature with flame +2 also receives extra dice against a ring of fire.
So there are different ways that zone attacks can roll extra dice.
Marked for death is another way.