Ok, so I calced Syndra's castle drop and found a scale for Nami's tsunami:
Syndra drops her castle = Even if her castle was a mere 200 meters above the ground, dropping it would result in an output of 1.96e+13 Joules, High-end Small Town level. However, considering the castle is in an unknown location somewhere above Ionia, It would likely be hidden within the cloud layer where no one would be able to see it. If she dropped it from that height it would result in 3.15e+14 Joules, High-end Town level.
Scaling Nami's tsunami to an actual tsunami is somewhat challenging as tsunami's change speed relative to their distance from the shore. If we scale her tsunami to a tsunami on the coastline, it yields 4.38e+11 Joules, Low-end MCB level. But if we scale it to a tsunami in the middle of the ocean it yields 7.60e+13 Joules, Low-end Town level. Which should we use?