The UnderwaterTunnel leads from the WaterShrine to the KingdomOfEsteria. You have to be wearing the OpalArmour to get through here.
PeterHull: There must be something pretty scary hiding down here - but what?
PeterHull: I think the OpalArmour should be pretty poor in 'normal' combat (Opal is a very weak material!) but it has some special powers. Maybe light reflection?
TeamTerradactyl: How about when Opal touches water, it creates air? That's why you have to wear it the whole time you're down there. Instead of some boss, it could simply be shifting rooms and a very detailed and confusing maze with scripted enemies (like the GhostMiners in OpalCave). Remember that the party has leveled up enough by now to survive pretty intense battles.
A proposal from ChrisBarry:
The character with the Opal Armor heads down the tunnel
alone. Returning will reunite you with your partner, which just bunks with
the Oracle while you're gone. The Underwater Tunnel itself will be a
straight left-to-right affair (or whatever direction the Kingdom is), with
erratic walls like the Opal Cave, but still generally straight. We can put
chests down here with mostly big money (sunken pirate booty!). Enemies are
here too; it's basically an endurance run with enemies and no boss. Exactly
where you come out at the end of the road is up in the air right now.