The only area on the north shore of Cape Cod itself where I have done
much collecting is Barnstable harbor.  This area is somewhat
different from the tidal areas of the south shore, but it is also
different from most of the Cape Cod Bay (north shore) coastline.
Because it is a long, sheltered, rather shallow bay, the wave action
is greatly reduced, and the temperature is warmer than other areas of
the north shore.  But it is still colder than similar sheltered bays
on the south shore.  As you would expect in a sheltered cul de sac
without much wave action, many of the beaches here are muddy or muddy
sand.  Quite a few of the south shore species are still present here
(Ensis, Petricola, Nucula, Mytilus, Crassostrea, Mercenaria, Gemma,
Tellina, Nassarius trivittatus, Nassarius obsoletus, Littorina
littorea, Crepidula fornicata, Crepidula plana, Urosalpinx cinerea).
But other south shore species are absent, or rare (Anadara, Anomia,
Laevicardium, Tagelus, Epitonium, Eupleura, Anachis, Mitrella,
Busycon, Nassarius vibex, Haminoea, Polinices duplicatus, Crepidula
convexa).  And, other species which are not found on the south shore
(or which are found only in deeper water) occur intertidally here
(Siliqua costata, Macoma balthica, Modiolus modiolus).
To really get a taste of cold water New England collecting though,
you have to leave Cape Cod and drive a few miles up route 3 from
Sagamore, at the east end of the Cape Cod Canal.  My favorite
location is Manomet Point in the town of Manomet.  This is a typical
New England cold water, surf-washed, rock and seaweed habitat.  Huge
boulders are everywhere, covered with shaggy coats of Fucus and
Ascophyllum seaweed, with patches of sand and rocky tide pools
dispersed among them.  The water, even on a hot summer day, numbs
your feet in about 10 minutes.  Snorkeling requires a wetsuit, to
avoid hypothermia.  In the tidal pools, Nucella lapillus, Acmaea
testudinalis, Littorina littorea, and Littorina obtusata are the most
obvious mollusks.  Shake a couple of handfuls of rockweed into a
bucket and you may get 50 or more L. obtusata, in a variety of
colors.  This location has the largest Littorina littorea I have
seen, up to about 35 mm.  I have found Margarites helicinus and
Margarites groenlandicus once in a while.  Lunatia heros and Lunatia
triseriata can sometimes be found in the lower tidal zone.  Even
Buccinum undatum and Colus stimpsoni occasionally end up stranded in
the lower tidal zone, though they prefer deeper water.  Large
Modiolus modiolus attach to the bases of boulders.  Hiatella arctica
can be found under rocks in the lower tidal zone, and Mesodesma
arctata in sand pockets.  Brittle stars, blood starfish, sea
cucumbers, green crabs, and green sea urchins are also common.  There
are also some great species just offshore, which unfortunately are not
found intertidally, like Calliostoma bairdi, Aporrhais occidentalis,
Neptunea decemcostata, Placopecten magellanicus, and Chlamys
islandica.  The vertical tidal range here is about 8 feet, and once
the tide starts coming in, it comes in pretty fast - not as fast as
the Bay of Fundy, but you still have to keep your eyes open.  More
than once I have lingered a bit too long, and had to wade through a
waist-deep channel of frigid water to get back to shore.
Hope you enjoy Massachusetts.  It's where I lived most of my life,
even though I'm in Rhode Island now (just a few yards south).
Regards,
Paul M.