That is an out-of-the-box stock game issue, which we assumed initially was caused by an improper use the Terrain Extractor. In efforts to "fix" that steep fault line, all we succeeded in doing was to make matters worse. It seems to be a bad "seam" in the ocean floor there. No eye witness accounts of any steam vapors - at least, none that have survived...
The "artwork" denoting the shoals is crude, and some one of these days, I'll attempt a refinement of the artwork. However, the game is also not very helpful in that regard either. Getting something sized correctly for the area, and yet have it be visible from courser zooms is nigh on impossible. Therefore, for now, the entire terrain grid area is covered. An earlier poster in a different thread called it the "Morse Code Line".