I can't speak for most triathletes, but all the ones I know have a good working knowledge of the difference between SCY/SCM/LCM and open water. It is obvious that a lot of folks dont take into consideration the actual distances on OW courses, but with the advent of measuring devices, they can finally get some good comparisons of apples to apples.
And triathlon races are measured all over the place, 750 yards or meters, 1500 yards or meters, 1.2 mile, 2.4 mile, etc. In this country at least(and wherever WTC has races) it is metric and miles...But perhaps you have a different crowd around you...(-;
The only thing I use to see how I swam in open water events (i.e. stand alone swims or triathlons) is how I finished relative to people & names that come up in all of my races.
I have a section of the bay here in Melbourne that is approx 300m long, and although I roughly know what times I can/should do for each interval, even the slightest ripples make a difference to the time.. so with a wetsuit it is anywhere between 4.10 & 4.45, depending on conditions & how crooked I swam. Without a wetsuit is normally 4.30-ish to 5 minutes. And of course...one direction may be 10-15 seconds faster due to wind conditions & direction of the chop.
Regardless - it's awesome to swim out there & not have to worry about bumping into people (though my shark paranoia is with me at all times....)