I would encourage the students look for shell, and to sketch them in place. Not picking them up, just sketch them as they rest on the sand...even if the water tickles your feet.
Original Conch Shell sketch on old book page by Yoopershop, $25.00
Julia Wright Jewellery, mussel shell drawing
Original watercolour painting shell study III by SeasideStudiosUK
23 creative sketchbook examples to inspire high school Painting / Fine Art students
seashell drawings | Art: SEASHELL ABSTRACT II by Artist Marcia Baldwin
Julia Wright Jewellery: Natural forms drawing, mussel shell
Student Art Guide: International GCSE Art Sketchbook Examples
organic form <-- I love sketchbook pages like this, full of studies and notes.