Easiest place to launch and fish is out of Swan Island boat ramp. The ramp itself is at the back of the harbor, there is a decent dock and there are fish in the harbor (not current at all), as well as out in the current (which is usually quite slow compared to up around West Linn and Oregon City). It is definitely where I would try first. There are usually folks out there fishing so you will be able to find spots by observing. In general, the biters int he harbor tend to move around (you will mark fish everywhere, but they will not all be biters).
I like to start around the first big dry dock (which is on your left as you head out) and fish from there out. If I do not get some action pretty quickly I keep moving. In any given spot move your casts a few times. If you do not get any bites then move spots. Use anchovies or herring and do not waste time changing baits - it is just like real estate: location, location and location. If you have a hard time find biters then head out into the main river. I tend to like to stay closer to the North/East shoreline (on your right as you head out). Just head down towards the railroad bridge and stop whenever you mark fish near the bottom. Out here you will not mark the fish densely like back in the harbor, but if you find them they generally bite pretty well.
Last thing to remember is that the Willamette is tidally influenced all the way up to the falls. Generally the best bite out in the current occurs during the maximum current flow. Depending up on the Willamette river level vs the Columbia level that can occur on either the incoming or the outgoing in this area. Look at this site to see whether the best flow is on the incoming or outgoing:
http://or.water.usgs.gov/will_morrison/monitors/
Use this site to check the expected levels and observe the tides up and down the river:
http://water.weather.gov/ahps2/river.php?wfo=pqr&wfoid=18685&riverid=203289&pt[]=142028&pt[]=147483&pt[]=142704&pt[]=144162&pt[]=142246&allpoints=144254%2C141263%2C142810%2C144173%2C144257%2C142028%2C147483%2C142704%2C144162%2C142246%2C142980%2C141602%2C144049%2C144431%2C144435%2C144243%2C146799&data[]=hydrograph
Best of luck and maybe I will see you out there one of these days.