Over the years Team Asha Triathlon has introduced swimmers to Open Water Swimming (OWS) at Gull Park. It is a great place to start out on your open water experience.
Advantages: With the closed channel, calm waters and plenty of sighting markers to choose from, this is a great place for first timers to get comfortable and learn the basics of OWS. The zero visibility underwater is also an advantage as it helps new swimmers re-calibrate their equilibrium and gradually overcome the feeling of nausea (or sea sickness). Water is tested on a weekly basis.
Disadvantages: The water looks, well, 'Yucky' - with the occasional blob of duck poop floating around (it could be vegan or may be veganish - I don't know, I haven't tried it). But it's all good. In the main channel you have to be careful and look out for kayaks and boats. Having a swim buoy (orange/yellow) and a bright colored swim cap will help make you more visible to the boats/kayaks.
Word of Caution: The lagoon is shallow and in some areas close to the shore, if your stop and stand, you could cut your feet on the abrasive (shell strewn) bottom. Further, don't "hug" the shore as you may stub your finger on a parked boat or the pier.
The lagoon is maintained by Foster City and is frequently drained/flushed as rain water from the streets drain into the lagoon during the rains. We typically do not swim for a few days after a heavy downpour as the drainage of water into the lagoon could have increased the bad bacteria levels. Be safe rather than sorry - avoid the lagoon during the rainy season. Aquatic Park in San Francisco or Cowell Beach in Santa Cruz are better places to swim during the rains.
Lagoon water is tested once a week - San Mateo County has an interactive map that shows you if the water is good for swimming or not. Waters are tested on Monday and the updates show up a day or two later on the sites below.
Interactive map. Just shrink the map to see the entire area covered by San Mateo County testing. 'Green dot' means good to swim, 'Red dot' means avoid swimming.
Water quality webpage: The link to the interactive map (above) is on this page.
The rather unnatural blueish green color of the lagoon is because the city dumps 100s of gallons of non-toxic blue dye into the lagoon in summer to help stop the growth of algae. Here is an article about it.
I've used Google Earth to map some of the more common swims we do at Gull Park. Here are some photographs and specific notes below each one.
A swim from the beach to the 'Yellow house' and back (Ghar wapsi) is about 400 yards. Always stop before crossing the main channel and look out for boats and kayaks. The main channel is the water body in front of the Yellow house - the turn around point. Sighting tips - the yellow house bang in front of the beach (they may repaint it some other color someday - so don't hold me to that 'Yellow House'). On the way back aim for the big trees right behind the beach.
This 200 yard swim is within the enclosed area near the beach - a good session for sighting and turning in open water. Sighting tips: There are always boats parked there - you can make a triangle based on them.
Right side bridge and back is about 1600 yards. Good for Olympic and Half Ironman distance training. We typically have the swimmers stay close to the shore (stay right going to the bridge and right on the way back). This keeps them off the center of the main channel which has kayak and boat traffic. Keep an eye out for oncoming swimmers as they may not be keeping the same traffic rules as you.
Left side bridge and back - about 3200 yards. Good for longer swims and for the full Ironman distance swims as well. Again, stay close to the shoreline (not too close) and avoid swimming in the middle of the channel. Traffic rules are NOT followed by all swimmers so keep an eye out for oncoming swimmers.