I-90 and Blewett Pass is the fastest and easiest way to get to Leavenworth.   Plan on a 2-2^1/2 hour drive.  I-90 is 3 or 4 lanes each way all the way to the summit of Snoqualmie Pass where it drops to 2 lanes each direction.  97/970 is a country 2 lane highway with occasional passing lanes.  There are several trucks.

From Seattle, get on I-90 heading East over Snoqualmie Pass to Cle Elum.  Take the 2nd Cle Elum exit, #85, and follow the signs East to Wenatchee.  You will be heading East and then North on 97/970.  You will cross over Blewett Pass and descend to the Wenatchee River and Highway 2.  Turn Left on Highway 2 and follow it 5 miles to Leavenworth.

As you are descending into Leavenworth you will pass Safeway and Marson and Marson Lumber.  Turn Left on East Leavenworth Road and travel .6 miles to the City Park Boat Launch area.  You'll see your signs and equipment.  Park where directed.