Edge Detection is used in new 0.18 Factorio Saves, to reduce the save/load time.
Older Saves, started with the mod version 0.6.XX and below, will retain the data for already found FluidAreas, new FluidAreas will use the new method. To use the new method on all areas, you would need to use the /RestoreWater command to restore all non-removed areas, then redeploy your offshore pumps. This would effectively reset the FluidAreas.
Long Save/Load Times
If your 0.17 save takes too long to load, use the setting FluidArea - Remove From Table. This will remove all depleted FluidAreas from your savefile. You'll no longer be able to refill or restore those areas. This setting can be used in 0.18+, but the edge detection should make the save files much faster to save/load.