They'll be fine - I used to swim in water that temperature (or colder) during the summer! The boys have it rougher though ;-).
Extended exposure to cold water or hypothermia can weaken your body's defenses (as it lowers your temp) but it isn't a major cause/effect that being cold or cold weather makes you sick.
Extended exposure to cold water or hypothermia can weaken your body's defenses (as it lowers your temp) but it isn't a major cause/effect that being cold or cold weather makes you sick.