There are 5 missions which take place in the instanced Thalassius. Three of these, Evidence of Unity, Sea serpents, and The Hydromancer are part of a story arc which needs to be completed in order. There is no extra reward for completion however.

• Locating the Statue (P.2)
• Coral Diving
• Evidence of Unity
• Sea Serpents
• The Hydromancer

The non-instanced Thalassius has 5 loot dropping named.

• Kraj Iurolapos - Supposedly this kedge can spawn in place of any yellow+ con "experimental Kedge" mob. A known spawn point is listed below. Summons.
  Broken Compass -- Glowing Seaweed Amice
• Rockcrusher - This crab is a random spawn from any mob outside the tunnels. Not only is he a loot dropper, but he is the target for "Reacquiring Control", a task given by Sorcerer Xycl. Does NOT summon.
  Polished Fire Coral -- Polished Ragrua Fang Ring
• Muckdiver - This guy is also a random spawn from outside the tunnels. Like Rockcrusher, Muckdiver is for the "Reacquiring Control" task. Summons.
  Muckdiver's Eye -- Muckdiver Fin Earring
• Wavecrasher - A named golem which spawns inside the tunnels. A known spawn point is listed below, but may have multiple.
  Cloak of Crashing Waves -- Mask of Crashing Waves
• Phyrael Mollotus - This kedge named spawns randomly from blue con mobs in the cave system. Summons.
  Phyrael's Belt of Coral -- Phyrael's Coral Collar

Because of the random nature of these mobs map points aren't as useful, so I'm not adding any.