To answer your primary question, eating before swimming does not cause cramps.
Whether oxygen-deprivation stomach cramps are real or not is open to debate, as is whether they can be brought on by eating right before engaging in strenuous activity. What’s not in doubt, however, are the lack of deaths associated with swimming right after eating — there hasn’t been so much as one drowning attributed to this, not even a near drowning. For something that was supposed to be fatal if you so much as dipped a toe, this particular old wives’ tale proved to be a dud.
So what to do with this information? Those who are very careful about not taking any risks that are avoidable might want to put off striking out for the other side of the lake right after having tucked into a large meal. However, ordinary levels of swimming or just general horsing around in the water need not be eschewed, even if a whole turkey has just been consumed, feathers and all. Likewise, strenuous sidestroking can still be safely undertaken after a snack or a light to moderate-sized meal.
As regards heart attacks, entering cold water can cause a heart attack or lead to accidental inhalation, but it's a rare occurrence which has nothing to do with eating.
In humans, cold shock response is perhaps the most common cause of death from immersion in very cold water, such as by falling through thin ice. The immediate shock of the cold causes involuntary inhalation, which if underwater can result in drowning. The cold water can also cause heart attack due to vasoconstriction; the heart has to work harder to pump the same volume of blood throughout the body. For people with heart disease, this additional workload can cause the heart to go into arrest. Inhalation of water (and thus drowning) may result from hyperventilation. Some people are much better able to survive swimming in very cold water due to body or mental conditioning.
This involves freezing water, and sudden immersion. If you're not falling through ice, it's probably not cold enough, and gradual immersion helps to prevent it.