Severe cramping entire both legs

I do not know exactly what happened to you, but this explanation seems highly unlikely.  The human body is typically more than capable of keeping itself in 'balance' even if you drink a fair amount of water.  Especially since you were also taking in (what sounds like plenty of) salt.

How hard were you riding?  How is your swim fitness?  How was the heat/humidity?  Those things seem the more likely culprits, but there's always the possibility of some other issue.

I do not know exactly what happened to you, but this explanation seems highly unlikely.  The human body is typically more than capable of keeping itself in 'balance' even if you drink a fair amount of water.  Especially since you were also taking in (what sounds like plenty of) salt.

How hard were you riding?  How is your swim fitness?  How was the heat/humidity?  Those things seem the more likely culprits, but there's always the possibility of some other issue.

RazorTri - off topic - I noticed you had a hip replacement, my wife goes in for hers next Tuesday.  She was curious to know how long it took for you to recuperate to the point of being able to begin training again?

4-5 pounds is definitely on the high end, but not unheard of (Alberto Salazar was in the 6 pound range).  When you train in the heat/humidity how much of these fluids do you replace?  Although, the gut is trainable, I would assume its pretty hard to get in 100oz of fluids/hr.

I will use myself as an example, because I had a similar experience to you.  I also have a highs sweat rate from 3.5-4 pounds.  I am a salty sweater.  Race week, and before long training sessions, I never drink plain 'ol water.  I have it with NuuN or saltstick or plain table salt.  It sucks, but I learned that this is the only way I can get through longer hot/humid efforts.  I do the same on race day.  I also follow the Bob Seebohar (sp?) method of sodium loading before a race (I can email to you, if you are interested).  According to him, you can't just take in a bunch of sodium in one sitting (like chinese food, etc or even a bunch of saltsticks..and my experience, too).  You need to take it in over time to build up your blood/sodium volume.

During the race yesterday, I took in 60oz/hr (I trained for it) on bike and about 45oz/hr on the run.   It was brutal yesterday, no matter if the temp "only" read 88 deg.  It was humid, and humidity is not our friend (sweat is not able to evaporate to properly cool your skin).  I also took in 1200mg of sodium (and other electrolytes/hour).  I still think I can take in more.  Even though it was a tough day, I was still able to take 3rd in my AG (I have never podium'd in any IM event).  Yes, slower than I would have in cooler temps, but was able to finish strong and more important to me....I went to the bathroom during the race (in T1) and an hour afterwards.

Since this hasn't happened before and you live in Arkansas and probably train a bunch in the heat, I think you were depleted at the start and your body was asking for electrolytes.  Lots of differing opinions out there and the science of it all (Tim NOakes, etc.), however, most of these people don't sweat like us 3+ pounds/hr.  Its a different ball game.  I was lucky enough to speak with Craig Alexander on the topic and he only sweats 1.5-2 pounds/hr in Kona.  He only takes in 1 liter (33oz)/hr and 800mg of sodium per hour in Kona.  I have tried that before and ended up in bad shape.   Look at how much sodium Jordan Rapp (google his race nutrition) and you will see the other end of the spectrum.  Everyone is different.  I suspect you are on the side of needing more.

Again, my anecdotal experience (but I know of others), and I believe your coach is right and will need to look at taking in even more electrolytes in IMKY AND properly hydrate the week of the race.   I will share with you what I took in yesterday, if you are curious.   

Sounds like lost electrolytes to me too.  I am a very salty sweater.  My bike helmet looks like a coke addict's paradise after a long bike ride because the strap is caked with salt.  I can literally scrape it off.

I have numerous times lost 8+ lbs in 3 hour bike rides even after consuming 40+ oz of liquid.  I have to take 3 Saltstick pills/hr when racing HIMs.  And I have to combine that with electrolyte drink too.  I thought over time I'd be a more efficient sweater, ie less sweat.  But it seems the better I get, the more I sweat.  The more I sweat, the more electrolyte pills and drink I have to consume during training and racing.

Since you're used to that level of training, it seems surprising that the level of cramping you had was above normal.  However, that doesn't take into account the water consumption during the week.  Peeing once an hour is very high.  

One of the things about hydrating before races is that if you're peeing once an hour and the urine is clear, you're actually doing nothing but flushing electrolytes from your system.  And it winds up being liquid in, liquid out, with no hydrating benefit at all.  The point of hydration is to retain the hydration, which means you shouldn't be urinating so much.  I've changed my hyrdation strategy from drinking lots of water during the week before an HIM to consuming more fruits and vegetables.  With fruit and vegetables being mostly water, what you get is the benefit of hydration, the natural sugar, complex carbohydrates, and high quality fiber.  And since it's not pure water but rather a combination of those things, you wind up retaining more water and hydrate more efficiently.  For me, now it's lots of oranges, grapes, spinach, etc.  Plus, it tastes a lot better than water and even G2.

Just came across this site and this was one of the first threads I found.

Razor - this is exactly what happened to me at Racine and my coach and I nailed it down to too much hydration leading up to the race along with a nutrition fail on my electrolyte mix. Not necessarily hyponatremia but flushing out too much of the important stuff and due to the heat my body just couldn't replenish fast enough.

I was in the same boat as you - last wave - had an awesome swim which I thought was going to be a precursor to a great day then I hit those headwinds and it was game over by mile 25. Left leg locked up so I pulled over to the aid station and worked it out with fluids and stretching. Made it to mile 42 then both legs locked up and it looked like I had baseballs in my quads. I kept trying to work them out but then my whole body cramped up. Quite the mess. Ended up in the hospital with the scarlet letter (DNF).

When I was at the hospital later they ran some tests and I had low potassium. Took about a week to recover but hitting it hard again for Branson 70.3.