Aquatic terrain?

[Dannyalcatraz]

A ship graveyard/reef of wrecks could come into existence and grow because of a variety of reasons:

1. Deliberate skuttlings
2. A creature or creature that preys on ships
3. Bad weather + reef/rocks
4. Xenophobic/territorial creatures

Such a conglomeration would be like a semi-submerged dungeon, home to all kinds of foes (natural and unnatural), concealing all kinds of treasures.

For a nifty example of this, check out the last few books in David Brin's Uplift Saga which has such a feature, albeit with alien starships.

Various incarnations of ST and SG:U also used this.


(Also check my sig.)

 

[Aeolius]

A ship graveyard/reef of wrecks could come into existence and grow because of a variety of reasons...

The PCs in my undersea game are beneath such a place, The Jungle of Lost Ships .

One might also have an interesting encounter with a castaway prison hulk.

Also, Shrieking Eels:

 

[Tonguez]

Haven't see oil slicks mentioned
or areas of slimey seas either sticky or corrosive crawling with things that should not be