They attracted my attention not only because they were so noticeable, but also because I have been doing shorebird surveys in the same area for over 10 years now and I didn't recall seeing these egg masses before.
One was bright yellow and wrapped around algae or seagrass:
The other was bright white, shaped like a corkscrew, and embedded in the sand:
I finally tracked down the snail making the yellow egg masses. It was a bubble snail, Haminoea vesicula. Here are two images to help you visualize how the yellow egg masses consist of rows of developing embryos:
I showed examples of adult bubble snails on 4 June 2015, so check out those pictures here.
I kept meaning to return to find out which species had produced the white corkscrew egg masses, but hadn't found the time. Today Jeff helped me out — he identified the distinctive egg masses as belonging to Rictaxis punctocaelatus.
Below are two close-ups of these spiraled egg masses:
I haven't seen adult Rictaxis yet (it's been on my wish list!), but if you'd like to see examples of these wonderful local gastropods, check out pictures here.