Being A: given the advice to drink lots of water, and B: being unaware of hyponatraemia can lead to obsessive over-consumption as an attempt to "be safe" and prevent dehydration. Educating users on hyponatraemia and encouraging them to drink electrolytes is the easiest way to fix this, since MDMA won't interfere with their awareness of these issues while they're high.

Hyponatraemia is certainly a risk of MDMA use, but it's rather different from the "I'm fine, I'll have another" lack of self-awareness of alcohol and so on.

For what it's worth, this rather depends on the drug. One of the major problems with alcohol use is that being drunk characteristically impairs one's capacity to determine how impaired one is.

With some other drugs, quite the opposite is true. With most hallucinogens, for example, one is very much aware exactly how impaired one is. With enactogens (Ecstacy/MDMA, mephedrone, etc) a similar thing happens, except that obsessive self-monitoring is characteristic of their effect, so users tend to be more than usually aware of their own condition.