HMCS Bitter was a pirate ship hidden in Blood Omen: Legacy Of Kain. It remained undiscovered in the games code until a project to recreate Blood Omen in 3D uncovered it in early 2010.
The Bitter can be accessed from a secret door, behind "Cap'n Bitter's Sudz" in Vassebunde. (The Door is out of view and requires gameshark codes or PC trainer to access). The door leads to a teleporter, which goes onto The Bitter itself; a large ship complete with pirates and what is presumably a sea-battle with another ship.(Though this could be interpreted as The Bitter itself in two different time periods) The section finishes with a return to the shoreline with pirates burying treasure.
It is unknown what caused developers to remove access (this part of the game is still present, just access has been blocked) to The Bitter, it is possible it was just considered 'out of character' with the rest of the game.
HMSC stands for "Her Majesty's Canadian Ship" - a nod to Silicon Knights home country.
In Legacy of Kain: Soul Reaver, there is a similar (albeit much smaller) ship in an almost identical location within Nosgoth, this may be evidence that Crystal Dynamics was aware of The Bitter