Burnt 1787 in body pump, is this right??

for that short of a time, i would say definitely not accurate.

i have to do aerobics for close to 90 minutes to get a 1000 calorie burn, and i weigh 318lbs. i also have a Polar FT6 HRM with a chest strap so I get a mostly accurate reading.

definitely change the batteries, try to recalibrate it, contact the company to ask questions regarding the high number, or probably the best one, invest in a HRM with a chest strap for more accuracy.

As everyone has explained it is wrong. Along with possible battery issues, if there were a lot of fans in room it's possible the electronic interference also created the false reading. My older garmin used to go nuts next to a big fan.

Incorrect. I wear my HRM while swimming and get readings just fine. There are HRMs out there that are waterproof and you can wear them during water aerobics/swimming. I personally own both a F6 and F4 and both work in the water.

Waterproof and safe to wear in the pool does not mean it is able to transmit uninterrupted through water.

The instructions for both the F4 and F6 both say "may be worn when swimming", they do not say they will give you an accurate reading when swimming for the reason explained here www.heartratemonitor.co.uk FAQs:

Generally, HRMs do NOT work (read the heart rate) in the water due to inability to transmit the radio (FM) waves under water. They are designed to transmit in the air. It is possible to get a good reading in the pool only when your chest area, and the transmitter, is clear of the water. This is the same for all chest strap type HRMs.

http://www.heartratemonitor.co.uk/heart_rate_monitor_faq.html#swim

It suggests the (breathtakingly expensive!) Polar RCX5 for swimming as it operates on a dual frequency and switches to one that works fully underwater.

So you get a calorie burn at the end of your workout but that will be estimated from the intermittent times it could catch a reading. You'll also find high chlorine or salt affect transmission.

That HRM site is wrong for specific make/model then.

The FT7, and maybe cheaper FT6 and FT4 can transmit through the water between strap and HRM, and through water to air if hand is out.
Depending on factor of chemicals in the water perhaps, and how long your arms are.
I've tested with strap in water and arm fully out, just as it would on air part of a stroke. Reading was seen and changed, so not just a stuck last seen value.
Now, weak battery in strap definitely affects that ability to send a strong signal. When it was getting old, it would lose it on farthest stretches. And the HRM would read 0 pretty quick, it doesn't hold on to last seen value and assume that at all. Which actually for swimming would have been more accurate.
You can tell it lost it frequently when the calorie count is very low. That led me to go back into the water and see, and indeed it probably was blanking out each time the arm went out too far.

Now in my case I'm not using the Polar's guess of calories anyway because it's so off for me. I just needed the avgHR, which for infrequent readings is still accurate, as my swimming is consistent. So haven't replaced the batteries yet.