The problem with supercriticality is that due to the circumstances under which it arises, you lose any clear distinction like "liquid/gas" as you might get at the surface of a conventional ocean. Instead, you get a smooth increase in density and viscosity from the gas phase bits of the atmosphere into the supercritical bits, like a fog that just gets thicker and thicker. If the material in question can exist as a liquid under higher pressure, the smooth transition will continue, getting thicker and more viscous until you eventually end up in something that seems unambiguously liquid. That'll happen with hot, wet superterrestrial worlds, for example, but I don't think you'll get liquid CO2 on venus-like worlds.
You won't, therefore, have "oceans" of supercritical fluid... just banks of thick, foggy "cloud" that rapidly pour and settle into low-lying areas.
As for what it looks like, you tube does have some nice videos of supercritical transitions, such as this one. Watching the transition in process is interesting, but I'll include one possibly relevant screenshot:
From 1'48" into aforementioned video... this almost shows what I was really hoping for. There's liquid CO2 at the bottom (which is clear and shows some dark material behind it), and (I think) mostly gas-phase at the top (which is also clear, and shows some light material behind it), and the bit inbetween is some opaque and turbulent supercritical CO2. You can see it forms billowing cloud-like feature, both at the air-side and at the liquid side, showing how clear boundaries can easily disappear in this sort of situation.
So is an Exo-Venus with true oceans of supercritical CO2 plausible?
Maybe? We don't really know a whole lot about planet formation, after all.
How would the ocean look like, visually
Boring, cloudy and grey. And it wouldn't be an ocean, more of a fog bank.
The closest sort of phenomenon might be the terrestrial "sea of clouds"
(image source: _tiffany on flickr).
Note the ill-defined edges and billowing surface.
What does that mean? Like, the stuff dissolved in it? Presumably very vertically stratified, but other than that it could be all sorts of stuff. Too broad to answer here, really.
Would the oceans and coastlines look weirdly earth-like or would the surface be a bubbling mess of "soap-bubbles"?
Long term exposure to supercritical CO2 might be capable of causing surface erosion, as you'll still get equivalents of wind and waves in it. No massive destructive mechanical wave action (the density transitions are too smooth for that), but slow sedimentation under gravity and rippled landforms due to "wind"/"current" movement. Coastlines would seem likely to be smooth, with only fresh volcanic landforms or astroblemes showing sharp, complex shapes. I'm not sure that rain is likely to occur, so you probably wouldn't see classic drainage-basin type landforms, so mountain ranges upthrust by tectonic activity could have a very different set of shapes to terrestrial landforms, assuming that "ice" couldn't form on their peaks.
What other chemicals could be found in these oceans? What does the liquid CO2 solve? Will the oceans be salty? Acidic or basic?
You seem have segued from supercitical to liquid, there. If you meant "supercritical", I'll point you straight back to the wikipedia page you started from. It has stuff to say about its use as a solvent. I think the overall pH seems likely to be neutral. I'm not at all sure if a supercritical solvent could be saline... it seems possible. The upper transitional layers would presumably not have much in the way of dissolved chemicals in them, but the deeper you go the more you'd find.