During the last glacial period sea level was significantly lower due to the fact that a lot of water was stored within the masses of glacial ice covering the northern latitudes. So if one lowers the sea level and uses current bathymetry the former coast can be estimated.
As can be readily seen after doing that exercise, the outer coast would have been on the order of 40 miles further out from the present day coast. It also shows that a fair bit of what is now covered by the Salish Sea inland waters would be high and dry. But the story is a fair bit more complicated. The dry areas now under the Salish Sea would have been covered with ice and the mass of ice pushed down the land surface not only where the ice was present but for considerable distance away from the ice. Hence, former shorelines are found well above current shorelines in the Slash Sea. When the glacial ice retreated even the lower global sea level ocean inundated the inland areas that had been ice covered (isostatic-rebound-on-northwest-blakely and wave-cut-terraces-in-san-juans).
But something to consider on the outer coast is where would people have likely lived if they were in Washington during the ice age. Coastal settlements would now be under water. We do know that someone pushed a spear into the side of a mastodon 13,800 years ago (thoughts-on-manis-mastodon-and-western). Glacial ice still covered the northwest corner of the state at that time. Where did the hunter come from. One possible explanation that I tend to favor is that people traveled along the outer coast during the ice age. The map above would indicate that any coastal settlements would be lost, but some effort to identify sites where the combination of sea level and isostatic loading caused a stable shore area has been an area of ongoing research in British Columbia.