First off, what is an El Nino? At its root, El Nino is related to a relaxation of the trade winds that girdle the globe at near-equatorial latitudes.
A nice short description of global atmospheric circulation and the trade winds by Dr. Keith Meldahl of Mira Costa College
This relaxation of the trade winds, in the simplest sense, allows warm equatorial ocean water that is typically forced to the western side of the Pacific basin to "slosh" back to the east. And this has a variety of important consequences on our coast on the eastern side of the Pacific Ocean basin, including increased sea water temperatures and sea level. Sea water temperatures influence things like ocean productivity and the occurrence and distribution of organisms. In our area, El Nino tends to be associated with some pretty interesting appearances of rare or anomalous organisms.
The water level consequences, though, are what I want to focus on here. El Nino are associated with elevated mean water levels. For example, here are the average mean monthly water levels (relative to MLLW) for Port Angeles and Neah Bay (top panel below) as reported by NOAA's Center for Operational Products and Services:
So there are a variety of patterns of interest in the top panel - first you can see that mean sea level (this is the average of all recorded water levels over every month) varies over the course of a year, with higher sea levels in winter versus summer (on the order of ~20 cm higher in winter for Port Angeles and Neah Bay). This is why we tend to see more flooding in winter versus summer in our area, even though the astronomical tides reach very similar elevations in the two season. Your eye might also be able to register the pattern of falling relative sea level in Neah Bay on this graph, that is probably due in large part to vertical uplift of the earth's surface there.
The bottom panel shows the same data, but with the average seasonal water level pattern removed from the data in the top panel (and also only shows the time period for which the two datasets overlap)...so this essentially shows you how much mean water level is different from the long-term average. And this is where the two strong El Ninos in the record, in 1983 and 1998, really pop out (I've also circled them in blue). So during both of those events the PNW experienced multiple months of water levels that were ~ 30 cm above average...thats over a foot! That matters, especially in the winter when it gets added to the typically ~20 cm of elevated water level. The coastal impacts of the 1983 and 1998 El Nino are poorly documented in Washington State, but even the weak El Nino in 2010 caused concern in our area.
As of yet the strength of the El Nino that appears likely to develop isn't clear...but given the relative sea level rise patterns in Puget Sound, any extra mean water level added on top of rising seas is bound to get some notice.