Denizens of the Deep (OSS)

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: They say the sea is cold,
: And yet the sea contains the hottest blood of all,
: The wildest, the most urgent.
: -D.H. Lawrence, "Whales Weep Not!"
Despite their reputation for deadliness, lakes, rivers, oceans and seas fairly burst with life. These waters contain an incredible array of creatures. From the elegant simplicity of free-floating plant and animal plankton to the astounding grace of dolphins, whales and other cetaceans as well as the stunning colors of a coral reef the diverse impulses of life find full expression in the sea.
And yet, the marine world deserves its reputation as a dangerous place. Beneath the waves, in the cold, dark realms of the deep, life is a constant struggle for survival a deadly, never-ending dance between predator and prey, hunter and hunted. All marine life, from the smallest zooplankton to the mightiest sea serpent participates in this complex chain.
So, too, do adventurers who engage in underwater travel. As soon as such heroes enter the water, they become a part of the dramatic struggle for survival. They are both predator and prey, existing underwater at a distinct disadvantage.
This chapter briefly explores the diversity of underwater life with which adventurers can interact, and provides Dungeon Masters with the basic information necessary to populate their underwater worlds. In addition, a number of encounter tables, arranged by depth and geologic formation, can be found at the end of this chapter. These tables allow DMs to run random encounters efficiently during an underwater adventure or campaign.
* Marine Animals
** Gelatinous Zooplankton
** Crustaceans
** Arachnids
** Mollusks
** Fish
** Reptiles
** Mammals
* Underwater Encounters
** Encounter Distance
** Continental Shelf
** Continental Slope and Rise
** Abyssal Hills and Plains
** Oceanic Trench
** Seamounts and Guyouts
** Oceanic Ridge
** Coral Reef
** Rivers and Lakes
** Encounter Descriptions
*** Special Encounters
*** Underwater Hazards
* Dungeon Mastery Tip
** Encounters