High Heart Rate while swimming

It's likely a combination of things, but I had the same issue that I was able to overcome.

Swimming has a few basic parts.  You have the stroke or pull that's providing propulsion, you have your body that's creating drag and your position/form drastically effects the amount of drag.  Another factor often overlooked is breathing itself which can have a big effect on drag and overall oxygen levels and such.

What I discovered very early on was that I had a big breathing problem where I wasn't exhaling enough air as I swam.  I basically would take little half or three quarters breaths and over time i'd get more and more depleted of oxygen and my heart rate would increase and then all kinds of meltdown would ensue.  My fix was to count to three as I exhaled to ensure I got all the air out and pulled in a full breath.  After I get in the zone, then I'm fine but almost every swim start I half to count my exhales for the first few minutes.

Now for stroke and form, the best tool besides getting a good instructor is to grab a gopro or other waterproof camera and video yourself swimming both above water and below water.  Try to catch yourself in the middle of the pool to get a good "normal" stroke to see what you really look like.  Even as a non instructor type I could see a bunch of flaws right away watching the video.  I noticed I tended to push down with my hands on my pull which caused my legs to sink.  I had a weird scissor kick, i lifted my head somewhat each time I breathed and pushed with my opposite hand sideways a bit to rotate for the breath, and about 5 other things.  In a nutshell I was extremely inefficient in the water because I had a ton of extra drag.  To address the issues I simply hit youtube and looked for drills to fix each respective thing and worked on them for several weeks at a time.  It takes a long time and a lot of work, but it's well worth it and your efficiency will increase noticeably. 

Good luck and stick with it.

I don't understand how your open water 100 avg. is faster than your pool 100 avg.  That's not right.....and if it truly is,  then that's your answer to why your HR is faster is open water. (again, I doubt it)

Mostly, you are just not an efficient swimmer.....it's not complicated.  You are exerting more effort than your buddy.

You need more coaching to learn how to swim.

The ONLY correct measurements of your swimming will come from a pool with exact distance and lane lines.  You CANNOT compare open water times, in any way, to pool times, swimming at your level.  You need more coaching.  Nobody wants to hear that, and I get why.....but most of the advice that comes on this board regarding swimming is from people who have never been around fast swimmers and don't really know what correct swimming entails, much less how to tell someone else how to do it.

Hey, I swim with fast swimmers and watch them lap me over and over again 

They probably aren't that fast. LOL

Awesome, glad to hear it helped. 

It IS constructive....and I already said you wouldn't want to hear it....nobody does.  You have to do two things to learn how to swim.  First, you have to have someone who actually knows how to swim work with you on a regular basis. ...IN PERSON.  Second, you have to swim A LOT.