I doubt that mollusc watching will catch on as a popular hobby.
Some collectors (not many) do supplement their collection activities with
photography of living animals; however, both birds and butterflies are a lot
more "watchable" and a lot more photogenic than living molluscs, for a
variety of reasons.  First of all, they are much more active animals.  Their
graceful movements and (in the case of birds at least) wide range of
activities are interesting in and of themselves.  Snails and slugs are,
well, sluggish.  Record a bird's activities for an hour and you may have a
couple of pages of notes.  Watch a snail for an hour and you may see it
crawl a few inches!  Or not.  Watch a Spondylus for an hour and . .  well,
you get the idea.  Of course, Cephalopods might well be worth watching, but
most folks are not in a position to directly observe them in their natural
habitat.
        A second problem with mollusc watching is that humans are
essentially terrestrial, as are birds and butterflies.  Many molluscs are
also terrestrial of course, but the majority are aquatic, and watching or
photographing them means becoming similarly aquatic.  This is not beyond the
realm of possibility, thanks to Jacques Cousteau, but it is a bit more
involved than taking a stroll in a meadow.
        Thirdly, the color and beauty of birds and butterflies are best
observed in living specimens.  A mounted butterfly or bird just doesn't
compare visually to its living counterpart.  Molluscs on the other hand are
often downright ugly in their natural habitat, covered with algae, sediment,
marine growths, and self-produced periostracum.  Some do have colorful soft
parts of course, but the fantastic colors and patterns that make shells so
popular can, unfortunately, be appreciated only in dead specimens.

        Paul M.