Run to swim ok for heart?

I love swimming for a few minutes as a cool down after running or biking and don't have any problems.  It does depend on the water temp but my gym pools are usually in the 78 degree range so it's usually not an issue.  I'm more worried about it being too hot.

Now if you're saying you're finishing a 5K and then diving into cold water, that's another story, and I would talk to a medical professional about it (or just stop and stretch and cool down a couple of minutes first).

Way back in the day when I was lifeguarding, we'd go for a run (often around 12:30 in the midwestern heat/humidity) and then "cool off" in the pool before getting back in the guard chair.  

Of course we also didn't believe in warm-up or cool-down.  When you're 20, you can mostly ignore that stuff (queue Superman theme song).  We'd go from the chair to running shoes on to running.  The first half of the run was pushed for advantage and the race began when we hit the turn-around.  We could be back in the chair <5 minutes after the run and that included a fresh application of tanning lotion.