Here's some information with links regarding the past and current dangers of swimming in Hot Creek. First regarding the heat of the water and secondly, regarding the arsenic levels in the water....Petra

FAQ SHEET from Sierra Web:
http://thesierraweb.com/generalinfo/hotcreekfaq.html

HOW HAVE THE PEOPLE DIED OR BEEN INJURED?
As of December of 1994 the actual count of deaths or serious injuries is 28, of which 13 have been fatalities. The majority of these incidents have involved people engaged in an activity that is in violation of the posted regulations such as nighttime use, being in a closed area, or having a dog off leash. Many have also involved persons under the influence of alcohol or drugs.
WHY IS THE FOREST SERVICE SO STRICT ABOUT DOGS BEING ON LEASHES?
Three reasons: First, more than 70% of the people polled at developed recreation sites comment that they dislike unleashed dogs. The leash law is enforced to promote consideration for the majority of users. Second, here at @ot Creek at least 4 loose dogs have died in hot water when they fell in after wandering too close to the edges of pools. In two of these cases their owners were seriously burned and scarred for life when they jumped into the pools in an attempt to save the dOg3. Third, loose dogs can step into small hot steam vents as they cannot detect scalding conditions.as people can.

IS THE ARSENIC IN THE WATER BAD FOR YOU?
Based on water samples taken many years ago there were 50-60 parts per million arsenic in the creek above the hot spring activity. One mile below the hot springs the level was greater than 250 parts per million. The water will not meet safe drinking water standards and the long term effect of exposure to this element in the water is unknown. Arsenic is a naturally occuring element in hot springs water.

Regarding the topic of arsenic in the water at Hot Creek, it has been tested many times because the water becomes part of the water source for the City of Los Angeles. For full details see this link:
http://www.monolake.org/newsletter/00spring/13.htm

Naturally occurring arsenic from Hot Creek in Mono County is one of the main sources of arsenic in DWP water. Over the last five years, DWP has set up year round treatment at its Cottonwood Facility to reduce the arsenic levels in LA Aqueduct water. The result has been lowered arsenic levels, between four and seven ppb. Between 1980–1998 the average arsenic level was 6.9 ppb, although in April 1993 there was a spike of 73.3 ppb. If DWP is to get most of the arsenic out of the water, it will need to build a treatment facility at Hot Creek. DWP estimates the cost of a new plant would add $5–$14 to a customer's bill, but it is giving this option serious consideration because there would be multiple benefits—lower arsenic levels in Crowley Lake, for example.

** Note: Of the many articles I read none of them are consistent in units of measure. Some use parts per liter others parts per million and lastly; parts per billion. No articles indicated swimming in water which contains arsenic is a problem, though drinking some while swimming, even accidentally is likely, but probably harmless unless it was an everyday event.