There really isn't a launch at the west end of I-90. There is a little park, Day Street Park, I believe it is called. It is very small and you can only get car topper/carry-in boats.

You will want to launch at Sayres Pits, just south of I-90. To get to Sayres you need to get on Rainier Ave from I-90 (take I-90 exit off of I-5) and turn left on Genesee and follow your nose east to the water, go down the hill and turn left on to Lk. WA Blvd. and you will see it.
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Carl C.