Some people think it is going to be gradual but there are people who think it could be sudden for a few reasons.
Antartic ice sheets are melting at this time, but it is currently trapped by the ice there.
Most people don't think these sorts of things happen regularly, but glacial floods have been seen in the earth's history (as long as you believe the earth older than 6000 years).
Also, the reason that ocean sea level rise has not been seen everywhere up to this point is because the Arctic ice was over water, which meant that no water was added to the ocean. Antarctica and Greenland's ice are not currently a part of the ocean, so when this makes it to the ocean, things are going to go bad around the world.
Aside from the climate change aspect the more immediate problem is acidifcation and warming of the sea which has already killed off a quarter of the barrier reef and is having serious effects elsewhere with plankton.
And this is the other point, we have no idea what effects we are having on plankton populations. But then again, why do we need plankton, it isn't like we need to breathe.