Bread waits for no one - is this underproofed?

If I remember this correctly from my years in school (and it's been a few years), the "water ring" (or sometimes just a "line" along the bottom) has to do with the fact that the dough did not have enough time to "loosen up". The darker area is actually a strip of crumb compressed by the inner part of the loaf. It's probably baked, but because it is so much more dense in that area, it appears darker (or still raw).

Not enough proofing time (as well as a dough that might be a little too "stiff") can cause this.

If you'd really like to figure out the cause, it would be best if you posted the recipe and the exact steps (proofing times) if you remember. You mentioned that this loaf didn't proof as long, so that's most likely the problem. But, you also mentioned that you have had this problem before, just on the bottom of the loaf. So, if you post more details, we might be able to help you to tweak your recipe and keep this from happening.

Stephan

The dough did seem to have a lower hydration than I normally make (I think more flour was added during kneading!). I put it in the oven at 250C for 15 mins then turned down to 200C for the remaining 25-30 mins (I'm not sure when it came out as I asked one of my children to pull it out of the oven at time while I left for work). 

Lower-hydration doughs take longer to rise, so you need to give it more time to proof. Make sure you're letting it proof until a certain volume, and not just going by the clock. Also, you need to make sure the bread is done before pulling it out, e.g. by using an instant-read digital thermometer (they're not oven safe, so don't stick it in the loaf and leave it in the oven -- use it on the counter, after pulling the loaf out). Don't just bake by the clock.

Except now that I read over it again I notice that I don't put my loaf on a hot baking tray but a cool one. I wonder if that impacts how the loaf cooks - maybe the bottom takes longer to get to heat and cook resulting in that raw like dough line???

Sure it does. Get yourself a pizza stone, or a cast iron pan like the Lodge Double-play Grill Griddle, smooth side up, and put that on the floor of your oven, then preheat for a good LOOONG time, somewhere between 20 minutes and an hour, with an all-metal oven thermometer in there too. Watch it, so you can check to see whether your oven is really hot enough, and you can also learn how long it takes your oven to reach 250 C. Then when you load your loaf into the oven, load it straight onto the stone or iron pan. (You can use a baker's peel covered in cornmeal, and proof your bread on that, and slide it from that onto the stone or iron. If it's a sticky dough like pizza, you can put oven paper on the peel and then cornmeal on that, and the dough atop that, and slide the loaf or crust into the oven, including the paper.)

I'm quite new to baking and don't know what the inside temp of the bread was and I'm not sure how I'd know. Do you use a thermometer - what and when?

To use my instant-read digital thermometer (which is shaped like a round lollipop, the head having the display and buttons, and the stick being the probe), I remove the loaf from the oven, turn the IRDT on, switch it to Celsius or Fahrenheit as desired, remove the plastic sleeve that covers the metal probe, and insert the probe until the tip is in the 3D center of the loaf. I then wait about 45 seconds (it's not really instant) to see what temp it stops at.  Also, flip the loaf on its side and thump the bottom and listen to the sound (and feel the vibrations). You'll learn the difference between an underdone and well done loaf this way. I aim for 190F for soft sandwich loaves and 205 for crusty artisan boules, personally. Oh, and then move the loaf to a cooling rack, and DON'T CUT IT before at least an hour has passed.