and wanted to know if the boat was outside the low tide line
yes, but I don't remember him wanting to collect shells. We did collect ammonites and other fossils on holiday once. The trouble was that "sub-littoral" wasn't in my everyday vocabulary at the time either, so I probably resorted to the "You may very well be right" type of evasion that as parents we know we shouldn't. I soon found that it was much more profitable for both parties if I asked my son "And what makes you think that?" at which point he would quote chapter and verse from some book he had read, and I would learn a lot. I would also sometimes be able to expand or correct his knowledge.
Rod