Swimming in warm/hot water

that is the WORST. It is draining, simply draining. you need to drink lots of water/gatorade since you can't sweat and yet you are!  Jumping out of the water every once in a while and keeping a water bottle just to rinse you off helps too.  You do start to get used to it, but its virtually impossible to do intervals when its 85 degrees and above. When our outdoor pool gets that way, the coach keeps us at sets no longer than 200m b/c you really start to feel nauseous. Plus he sprays us with a hose.

78-80 is the ideal temp for sure.

Make sure you bring a water bottle, and it wouldn't hurt to put some electrolytes in there. You'll be sweating and not even notice it.

I used to get lessons at a pool in a swim club where the average member age was over 70, and the water was always in the 80's.

My pool is 82-83 and it feels awesome! I should mention that it's also outside, so the air is in the 40s at morning practice.

But really, if the spike in pool temperature seems temporary, I'll try to do longer and less intense swims. Lots of technique work, kick sets, etc.

If it's not - figure out what works for you. There were days a high school practice where the pool literally did hit 90. We'd do lots of sprints with lots of rest (think 4x50 on the 2:00 when we're all coming in on 30-40 seconds) and we'd get out of the water between 50s... and sometimes even hit the shower to cool off between sets.

Be careful, your bodies ability to dissipate heat is compromised in very warm water.

"Swimming in warm water becomes dangerous with long distances. In October 2010, a competitive swimmer died in near-90-degree degree water during a 10-km marathon. Water temperatures near 90 degree Fahreheit do not allow your body's heat to dissipate, according to Dr. Kenneth Kamler, author of "Surviving the Extremes: A Doctor's Journey to the Limits of Human Endurance." When interviewed by CNN, Kamler explained that this trapping of body heat can cause muscle spasms, possibly leading to heart arrhythmia or lung failure. Signs to watch for during warm water swims include nausea, headache and a lightheaded sensation"

90º and 100% humidity? That sounds like summer running in Florida. 

Definitely have a water bottle at the end of your lane.

Hmmmm…. my pool must be warmer than I thought.

When I lived in south Texas and we had five straight MONTHS off 100+ degrees outside with only outdoor pools they got pretty toasty.  Mostly I tried to swim in the evening when at least the sun was not beating down on me in 90+ degree water.  Water, maybe some electrolytes depending on the length of your workout, You get used to the temp after a while.  I'm in Fl now and have a nice indoor pool they probably keep in the low 80s and its all good   For some reason the noodle ladies always seem to get their way at most pools, probably because they are all retired and have nothing better to do but complain about the water temp. 

USMS Open Water Rules:

"302.5 Water Conditions
For swims of 3 miles or more, the swim, shall not begin if the water temperature exceeds 85° F""

FINA Open Water Rules:

"4.7 Water Temperature 
(a) The water temperature shall be measured 2 hours before the start of the race and must be a minimum of 16'C (60.8 F) and a maximum of 31 'C (87.8 F). The water  temperature shall be certified by the FINA Safety Delegate and the HMF/OC Safety Officer as measured in the middle of the course, at a depth of 40 centimeters."

I consider racing in water in excess of 85 F dangerous. The amount of risk depends on the swimmer health, the effort being made and duration of the swim. Clearly colder air temperatures might make it a bit safer, but I am in complete agreement with the USMS rules. The FINA rules were controversial when they came out. There were questions raised, and people suggested, that the high temperature limit was influenced by the temperature of professional races in the Middle East. I think training in water warmer than 85° is fraught with risk. Getting out occasionally to take cold showers or drinking ice water (with cubes) would probably make it a bit safer.