And I don’t just mean waves. What with things like planetary rotation, prevailing winds and marine currents, “sea level” is by no means the same everywhere in the world.
Why does this matter? Well, global warming is going to change a whole lot of things. Yes, in general, sea level is going to rise, but also currents and even wind patterns will change. As a result, the actual increase in sea level will be different in different parts of the world. I’m relieved to note that London and San Francisco look like they’ll do OK. If you live in New York, don’t look. (And I suspect something similar applies to Boston.)