First magicpaddler's question:

This is what Tim Noakes (Lore of Running) has to say:

Cramping, while uncomfortable / painful, is usually not medically serious. Risk factors include old age, undertraining, overweight, family history, and stretching habits. It does NOT appear to be related to dehydration or electrolyte levels. Excessive fluid intake (hyponatremia) can lead to cramping during exercise though. So if you're well-trained and not overweight, stretching especially before starting out might help.

marlin's question is harder. I'm guessing that the dizziness might have to do with low blood sugar - maybe if you eat something high in sugar, it gets rid of the dizziness, but causes an insulin reaction, which would explain the fatigue. See my long post on carbs somewhere in these forums. Maybe "pushing to the limit" uses up all your muscle glycogen and blood glucose, then you try to eat to compensate, but it's too late? Maybe try regular snacking (handfuls of GORP etc) while you're canoeing? I don't think it's related to electrolytes.

Good luck to both of you.