If the specs are identical the volumes are identical. Weight has nothing to do with volume.

Mathematically accurate, but would, for example, foam covered in laminate be able to float as much/more than a solid wood board with the same volume (ignore stability of said boards for the moment)?

I'm not sure how helpful any volume charts or recommendations are beyond being a very general starting point, since two boards could have similar volumes (as well as lengths and widths, for that matter) and much different stability.

Agreed. My frustration stems from the fact that for most boards I find a lot of ad copy instead of actual reviews, so I'm somewhat forced to rely on the manufacturer's stated starting point, which is sometimes wildly unrealistic either in a conservative or liberal direction.

A generalized chart for weight and volume for paddlers could be beneficial but there are many variables at play

Don't I know it! Hence the "demo, demo, demo" chant on the forums.

Maybe instead of a chart by volume, what we really need is a list of manufacturers who are conservative in their estimates, manufacturers who are realistic in their estimates, and manufacturers who have lost their marbles. 

I don't river paddle but I do go into the Gulf. Newbie at best, 8 times out. I was in 2/3ft waves today on my 12'6" Bic Wing and it handled the waves great. I was even trying to jump back to surfer stance for turns. (yep I got wet lol) Been on flat water with me, 6'1" 170lbs and my little girls about 55 and 70lbs no problem. lil heavy at 34 lbs but the way it's made it's durable as hell, I would imagine you want a tough finish in a river. Rocks and stones?

What about the shoreline and your launches?

Lengthy subject with a zillion points of view.

In the simplest of terms:

Stability and speed responsivness glide and other good news stand in opposition to each other.  Not just me, but the 'master shaper' opens with the same gag.

So what gives stability?  Width, length, volume, and surface area in contact with the water.
If you are serious about paddling outside of the surf, and you can get it up on the car, stock is the way to go (12'6")
31 is wide and stable, depending on the other issues mentioned.
28 or less favors response at the expense of stability (race boards).

We have the tools of area, concavity, blocky tails, and blunt entries to breed stability back into the shape and width.
There is a bottom line, Sea me.  Go to the races and try every board the manufacturers and dealers will let you ride.  The other option is to find a custom shop and tell 'em what you told us.

Just keep in mind, the hot board that makes you feel the glide is also gonna be a bit more difficult in the chop.

Oh yea, did I mention rocker.  Those numbers make a huge difference when the wind blows on the river.  More rocker, better performance in downwind chop, but less glide in the calm water.

I'll bet you are confused now.  You should be!  Be patient.  There are no easy answers.
If there are a lot of other people on your river, watch what they have. 
Width
volume
rail hard v soft
rocker
area (pointy or kinda parallel)

With all of that swimming in your head, you might just grab a demo and know its right.
But, please consider buying American made stuff so that poorly paid men can eak out a living.

So, Seame,

It was a good weekend to buy a board.  Whaddija get?

I've been playing about with a radical cockpit design.  Drop decks are the thing for racing.  So, I dropped the deck and put high sides, instead of the rolled edge.  Not quite as canoe like as the thing that Terrel built, but the cockpit replaces volume.  The sides are thin and allow you to get your feet to the edges, instead of on the rolled edge.  I like full freedom for feet to move about.

I think volume may be the most confusing element to shape.  I don't think many people really understand.  Of course, I'm not a surf shaper, so I'm just talkin about racing, training, cruising designs.

But, I see specs for weight and height to go with volume and I cannot help but think that some marketing people are talkin out their butts.

When you say choppy, do you mean choppy sloppy, or just choppy?  Another thing to think about is how tapered the board might be.  More parallel, more surface area, more stability.  Did you look at Yolo?  I think you will find them on your coast.

So, Seame,

It was a good weekend to buy a board.  Whaddija get?

I'm still weighing my options, I don't whip out my wallet to spend $1K+ lightly. I'm thinking about a Sirena 12'6" or something from the Fanatic family. I plan to have a chat with Jim K soon since he's familiar with both brands.

When you say choppy, do you mean choppy sloppy, or just choppy?

You're going to have to explain the difference before I can answer.

Another thing to think about is how tapered the board might be.  More parallel, more surface area, more stability.  Did you look at Yolo?  I think you will find them on your coast.

Other than magazine advertisements, I don't know much about YOLO boards and I haven't heard anything about the boards (yet) to convince me to trek down to the local dealer (over an hour away).

The same volume board will float the same weight, just remember that the board is part of the weight you are floating, so a heavier will be able to float that much less paddler.

Bingo! Now it makes more sense. Thanks.

Thanks for askin, Seame,

I am a garage shaper with my own ideas about shape for lakes rivers and sloughs.  As you are not likely to fly to my coast just to come out and play with me, I'll tell you what I know about the brands you mentioned.

Yolo is your coast, down south, Florida.  But, they have a presence here with a really cool lead guy that I like.  I got to hang out in the factory when they were building prototype boards.  I like the shape philosophy and the product.  As you are not buying a race board, I do not have the specifics to send you to the shop to see a model.

Fanatic, same deal, except I never worked on thier boards.  They are quite respectable and competitive with thier graphite race boards.  They are kicking some butt around the buoys.

If I could get you to my house to splash off my dock, I would put you on a 12'6" by 30 fast cruiser and see how you liked it.  Then I would take you to a venue that has comparable chop and slop to your river.  I got my wake up call at South Lake Tahoe Saturday to remind me of the difference between wakes, wind chop, and the slop that happens when the two combine.  I thought my race board did great.  If I only had a real racer to put on it, I might graduate out of the garage into marketing.  Ah, but alas.

I personally think that you, and most other consumer paddlers think too much about branding, price, and even construction.  Shape, shape, and shape should be your first concern.  No body else is you.  No where elese is your favorite venue.

My advice is to find someone who knows your typical conditions and will fit you to perfection. 
Then maybe consider weight in fiberglass for durability v pleasant to carry, resale based on reputation, and last of all, looks.
Price is important, no doubt about it.  I hate to hear about serious paddlers seriously considering imported boards.

We are a community.  We need to stick together.  If you can find a local dealer with connections to a local shaper, that is worth the effort to make you feel good about the system, even if it cost a couple of hundred more.  The local guys, if there are any, are your source for happiness, unless of course you know them and don't like them.

Jack, you make some great points, but I want to nitpick one. I think durability is very important. That may be because I paddle in very rocky areas, but it's paramount for any future SUP I buy. For most people it's not as much an issue, but it's probably number one for me.