How do I answer a question on the ethical implications of my work at PhD viva?
I'm not an ocean physicist, or anywhere qualified in oceanography, nor do I have a slight idea of what your topic is on.

That being said, research involving water, and any political ramifications that may impact it, typically will impact commerce related to the oceans.

In this case, theoretical commercial impacts:


Fishing. Your research may potentially result in a ban on fishing, or make it harder to obtain fishing licenses.
Trade commerce. Ships are still used to transport goods around the world, including oil. As above, any sort of research that indicates that the oceans may be unsafe or bars certain trade routes will result in economical impacts down the line.
National borders. Traditionally, countries own a specific amount of shoreline before international waters. A shifting shoreline may result in a changing border and or possible border disputes.
Tourism. Obviously, effects of the water, even perceived, may impact tourism.
Insurance. Rising tides = rising premiums.


Whether or not these apply to your topic, I can't really tell, but it's better to start with a list of things and then check them off as irrelevant gives the no ethical impact argument more weight.