I was looking all over for some and stumbled across some nice looking plastic adirondack chairs at Home Depot. They came in a few colors and were $14. They sit nicely on our baja shelf which is 18", about 6" of the chair is underwater so these may work for your shelf. I wanted plastic vs metal and they have been a great find.

Great! Are these lounge chairs or do they have ottomans or something with regular chairs? I saw where someone posted that they used these but cut off the legs to get the height they wanted in the water. I've seen some online that look great too but are out of stock so waiting for an email notification...(but they were very expensive and wouldn't want to cut the legs off!)

Not sure where you live, but the Home Depot, Lowes and Walmart in the Houston area all carry PVC colored adirondack chairs that are simple to adjust by cutting the legs.
Check the garden section.

They run $15 for adult sizes and $10 or so for kids.

I bought a couple of those PVC Adirondack from Home Depot for $10 a chair on closeout. Not sure if they list them online.