Intrigued, I went over to take a look. It was a beautiful California Red-sided Gartersnake (Thamnophis sirtalis infernalis)!
Sometimes this species has a wonderful greenish-blue coloration below:
Here it is pushing off the sand:
And one more — a close-up of the markings:
Many thanks to the family for sharing their colorful discovery!
ADDENDUM (26 November 2017): P.S. Eric and I were debating about the color of the underside of the snake. We discussed turquoise and teal green, but neither of them seemed right. Then Eric sampled the actual color of the snake in Photoshop, and we tried to find the closest match with a Pantone color (see photo below). We didn't find an exact match, but we found that Pantone 345 was closer to the snake than either turquoise or teal (although it still needs to be a bit grayer). Pantone 345 doesn't seem to have a name, so I guess it's up to us to make one up. What would you call it? :)