It's a shame that so many people are afraid to get their feet wet in Puget Sound. Maybe it's that the water temperature stays at about 48 degrees year round--rain snow or sun, much of the water in the greater Georgia Basin mysteriously stays cold and perceptively uninviting. The reason for the cold is the constant supply of deep, fresh ocean water that cycles ever so slowly down from the Gulf of Alaska; joined by the cool river waters fed by melting glaciers and rainstorm run off deep in the high elevations of the Cascades and Olympic Mountains.
Parksville, BC where nearly a half mile of sand beach is exposed at low tide to warm in the sun.
For those in the know, the cold water rule isn't really that consistent. In the summer, when the sun beats down on shallow sandy beaches before an incoming tide, the sea can get like bath water. Be it only for the first foot or two, it can still be very refreshing. In some bays around the sound, water gets trapped and cycles around with the meandering gentle tides. Water temperatures need to exceed sixty degrees fahrenheit for at least three weeks out of the year for oysters to spawn. So, any area that has an abundance of naturally growing oysters, you can bet on having a few weeks of warm water for the kids to frolic in--most of the oystering beaches are located in the South Sound.
Any big gently sloped sandy beach will produce a sufficient layer of warm Hawaiian-like water to dip your toes in, enough to get you in the mood for margaritas. Some great beaches for a mid-day sunny stroll on the sand are Alki Point in West Seattle, Dash Point State Park in Federal Way, Useless Bay on the South end of Whidbey Island--if you arrive by sea, Cultis Bay at the southern tip of Whidbey has nearly a square mile of walkable sand that gets exposed at low tide. Going northward into BC, one of the most famous warm sand beaches is in Parksville (about a third of the way up Vancouver Island's eastern shore)--the water there gets to be 80 degrees F on sunny days.
There are many beaches throughout the Puget Sound region that have shallow sandy topography. One easy way to find these beaches is to go to the Internet and use Google "Earth"--shallow waters are clearly visible in the satellite views in most areas. The best beaches will be the ones that are open to the weather versus protected bays. Shallow bays often harbor soft mud rather than sand. Beaches that are exposed to the relentless winter storms seem to have the organic matter wash out from the sand.
One of the fun activities for kids is skim boarding; a kind of surfing done by running and jumping onto a three-foot plywood disk that rides on a thin cushion of water at the shore's edge. Kayaking, paddle boarding and wading are all more enjoyable at these special beaches.