Signs on the Water

I feel free as a bird when out on the water in my boat.  However in the course of traversing waterways a boater will encounter occasional signs along the shoreline.  Most typical are signs notifying that making landfall is not allowed such as "No Trespassing", "Private Property", or "Keep Out".  Imagine my surprise recently when I saw instead a sign of "welcome" on a riverside tree...

Funny how just one word can change the whole tone.  The "friendly" sign informs via pictographs which activities are allowed and which are not.  Kudos to whoever designed it!

We boaters see many other shoreline signs serving to communicate a variety of information:

Some remind us not to litter...

...and that we're being watched.

Others alert us to hazards ahead...

...and ways to avoid them...

And still others remind us of what the consequences might be if we ignore those warnings...

The prudent boater is ever vigilant for such signs.

Recently, however, I encountered a different kind of signpost which used only an image etched in stone to convey its message...

On first sight I perceived it as possibly depicting a deer above and a fish below.  However, after consulting with folks knowledgeable in such matters, the term "Underwater Panther" was heard by me for first time.  In my subsequent search to discover more I've learned from  a 1960 paper by James H. Howard titled When They Worship the Underwater Panther: A Prairie Potawtomi Bundle Ceremony, that some Native American tribes believed there was a creature having supernatural powers that lurks below the water's surface: "Although generally considered an evil creature, the Underwater Panther is greatly feared and respected by all of the groups having the creature in their pantheon."

On the blog Burl Carvings and More by Steven Lalioff I found mention (in the comments) of the Lewis and Clark expedition having encountered such petroglyphs which the Indian guides found concerning, and that "the original meaning of the glyphs was 'Caution, one of these things LIVES here' ".
So if those I've consulted are correct and this is an image of the "Underwater Panther" it begs the questions...who made it?...and when?...and is there a situation where a warning is justified?

The other day while paddling on the Musketaquid or grass-grown river, this sign of a completely different nature was encountered...

A river crop circle?