Charts list sewage lines and of course at the end, their outfall. I have sampled at these outfalls for different research
projects. The difference in salinity (density of water) causes the water to rise. Some outfalls have diffusers which can run a few hundred meters in length, for those, its more difficult to spot a plume rising to the surface. The pictures look similar to sewage outfalls with short or no diffusers.
There are other reasons for places on the water where you see upwelling, which have already been mentioned. The most common occurrence is geography where you have currents and a rapid change in depth
In the SF Bay
there is a pinnacle near the GG Bridge and a ridge that extends from Angel Island in the Raccoon Strait...many others as well. Its fun to go right over them and watch the sounder and the hydraulics of the water surrounding them.