Boarding Torpedo
Boarding Torpedoes are (As the names suggests) are a type of torpedo used by the Imperial Navy to board enemy ships. Due to their role they act more like spacecraft than a weapon of war and thus, fulfills a role not too similar to the Shark Assault Boats. On the flip side of the coin however, Boarding Torpedoes are less maneuverable and armoured then assault boats, but are also a smaller target and thus more difficult to intercept. On the other hand, Boarding Torpedoes are seen in the new Space Hulk: Deathwing game and is where our fabulous Dark Angels embark on the dastardly Space Hulk. Due to this, we may have the first canonical picture of the Boarding Torpedo, looking more akin to giant, flying drill than anything else.
If it goes off course, however, you're fucked- you'll be sent screaming into the Void.
Battlefleet Gothic Armada
In Battlefleet Gothic: Armada, Boarding Torpedoes act as the smaller and more nimble cousin of the Shark Assault Boat. Space Marines are the prime users of these for some reason (Maybe for tactical and surgical deep strikes?).
While they are meant to behave like smaller Assault Boats in theory, in practice there is almost no difference between a Boarding Torpedo and a Shark Assault Boat.