Unknown Navigation Markers

[Grasshopper187]

On a recent outing on the Myakka river in Florida, I came across two navigation markers I hadn't seen before. One was an octagon painted white on the left side and red on the right with the letter "A" on it. I looked it up and found that it was a type MR, indicating safe water but there was no explanation of what MR means, why it was octagonal and why the colors were left-right instead of up-down. I assume the letter "A" is an alternative to 1,2,3... etc. but there was no "B" or other lettered markers around. The other marker was a square with red on the bottom and white on the top with the letter "A" on it. I couldn't find anything on that one at all.

Anyone know what these are and why they are shaped and colored as they are?

 

[alldodge]

http://www.uscgboating.org/images/486.PDF See page 7
All safe water markers (Octagon MR, Spherical, Can) are done left right and safe to pass on either side. The Letter or number found on them are there as a reference for charts for the area. If another was used at another location and not necessarily in the same local area it would have a B or other number on it.

The"Preferred" navigation channel markers are top bottom indicating which side is the safer to pass on (more depth or clearance).

If the square with a corner pointing up I would see it as a location marker (Can be found or charts)

 

[StingrayMike]

It might be a fairway day marker.
They dont have to be very the normal day shape design, just something that is appropiate to the area and location of the other bouys, and could have the letter "A" on it. If it had a light, it would flash the morse code alpha, designated as a fairway marker.
it is used to mark an entrance to a channel. That one would mean that there is safe water on both sides of the marker, but it is preferred to keep the bouy on your port side.