New to the area and want to find some areas to duck hunt by boat?

Are there any good lakes in the area or does it have to be salt water?

All the duck hunting I did (42 years living in WA state) was at Potholes Reservoir in Central WA, about 3.5 hrs away from Sea/Tac area.
Mardon Resort has a nice service called a "Duck Taxi" where you pay for 2 or more people, they take you out on their huge Taxi boat to a blind where the dekes have already been set up in advance by 30 min. They drop you (and your dogs if you have them) pre-dawn, then you shoot til noonish or after if the fly way is good.

There is a Jon boat on every location (camo'd) so if you don't have a dog, you can retrieve your ducks.

While opportunities are still there, they are becoming harder and harder to find.
Many clubs and/or groups lease huge chunks of land for their members to hunt.

My sons and I used to only hunt public land until 5 years ago.
We now lease a spot on tribal land.
No need to get up at 2am and be in a public area by 3am to secure a spot.

Our lease has been amazing.
So far this year, we are at 290+ ducks!!

Contrary to popular belief, not all leases break the bank.
Doing research and reaching out to land owners is the key to find something you like.

Sportsdad60's info and advice is sound and the Potholes still offer decent hunting.
However like most other public spots, it can get crowded.

Those islands are covered with plants that have large burrs that will stick to your dog’s coat. I cut a few dozen out of Huey’s coat with a pocket knife. He wasn’t amused.