Well, if you want to be right downtown, there’s waterfall park in downtown Seattle (international district). It’s very cool. I don’t know if you can have weddings there but Google it and check it out.
Snoqualmie Falls might be risky in the spring. . . Depending on the snow this winter, you could have trouble getting there.
If you are not dead-set on the waterfall thing and just want something pretty and “nature”-like, try the Graham Visitor Center at the Washington Park Arboretum. VERY beautiful, great location, and the staff are great to work with.

WHIDBEY ISLAND! No water falls tho. . That I know of. .

The only waterfall I can think of in this area is Snoqualmie Falls. I’m not sure how expensive that would be.
Sorry I can’t be of more help.

I got married at Snoqualmie Falls. .

It was free. . .

There is a big lawn area near the falls that is really nice! You just need to find the minister which isn’t expensive at all.

Salish Lodge