Underwater..

[divooneh]

Is there anything there besides clams and fin gleam?

[Celestial Wurm]

Almost every lake I've dove into has contained a chest or two. None have yielded great rewards yet, but I like to keep my hopes high.

[Elhana]

I've found that most smaller bodies of water (lakes and ponds) usually have at least one treasure chest somewhere in the water. Waterfalls often have a treasure chest hidden behind the cascading water. In ponds or lakes, look for a large rock outcropping or something else unusual and take a peek! You never know what you might find.

However, I've found that most rivers don't have a lot of interest along the river bottom -- but it may just have been my luck.

[Oathbreaker]

Would've been nice if they'd have stuck a skeleton at the bottom of a river by a fort with some kind of unique sword sticking through it to keep it pinned to the river bed, or if they'd have done something similar to that.