In the Northern Atlantic the water temperature can be very cold. At depth the water temperature will be about 0 degrees celcius all year around. Considering divers are spending sometimes upwards of five hours in the water at a time hypothermia is a very real concern. Not only that but hypothermia can aggravate DCS symptoms. To combat this problem divers wear drysuits with warm wool undergarments underneath. But the divers still get cold so instead of using the light helium to inflate their suits they opt to use argon which is a heavier gas ergo warmer inside the suit. But this means bringing an extra tank of gas with them on a dive. 

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The Rouses opted not to dive with argon because of the expense and extra gear needed. Did this decision cause them to be colder and in turn aggravated their DCS? very hard to tell but could have been a real possibility.