Had my 1st real wetsuit swim on Friday. I swam a mile in open water. My time was almost 20 seconds per 100/yards faster than my typical swim sets at that distance.

I was like a lightening bolt in the water. It felt super cool.

Downside: forearms were so fatiqued. They weren't sore, I wasn't tired, my shoulders weren't sore. I could have swam forever, but my forearms were just done.

Why were my forearms so give out? My stroke rate was about 4-6 strokes more, but I wasn't trying for that. It just is what it is.

Why was I so fast?

Even though I was faster, I prefer non-wetsuit swimming because of the arm fatique

Not sure what your ability is, but weak swimmers often have poor body position without a wetsuit. So jumping in a wetsuit they can see some pretty major performance improvements compared to a top swimmer who already has buoyancy in the water.

Had my 1st real wetsuit swim on Friday. I swam a mile in open water. My time was almost 20 seconds per 100/yards faster than my typical swim sets at that distance. I was like a lightning bolt in the water. It felt super cool.

Downside: forearms were so fatigued. They weren't sore, I wasn't tired, my shoulders weren't sore. I could have swam forever, but my forearms were just done.
Why were my forearms so give out? My stroke rate was about 4-6 strokes more, but I wasn't trying for that. It just is what it is.
Why was I so fast?
Even though I was faster, I prefer non-wetsuit swimming because of the arm fatigue.

It appears that in normal, no wettie swimming you are not using your forearms as much as you should be. Think of your hand and forearm as a paddle with which you pull yourself through the water. My forearms are often sore after a hard swim workout, along with most of the rest of my upper body. You were so much faster b/c the wettie works as a sort of super pull buoy for swimmers w/o good body position.

Did the suit force me to use my forearms more? It's a Roka Maverick Pro.

One other point: you might want to test your wettie speed vs non-wettie speed in the pool, where you can directly compare your speeds. Your almost 20 sec/100 yd diff seems a bit on the high side; i could believe 10 sec/100 yd but 20 seems a bit high. Is it possible that your OWS was current aided???

I tested it once in the pool, but got so hot after 200 yards that I had to get out of it. But in that 200 yards, the time was the same. In that pool test and the OWS, my time was 1:47/100yds. Without the wetsuit in the pool and open water I am 2:03-2:07/100 yards.

I tested it once in the pool, but got so hot after 200 yards that I had to get out of it. But in that 200 yards, the time was the same. In that pool test and the OWS, my time was 1:47/100yds. Without the wetsuit in the pool and open water I am 2:03-2:07/100 yards.

I like this explanation. The water was choppy. I love it when the water is rough because it forces me to focus on everything but the fact that I have to swim X distance. I am focused on the roll of the waves, breath timing, stroke timing, sighting and all that goodness.
I think I am just a wuss in the pool. My average HR for that swim was 125bpm. In the pool, my average is right at 108bpm., both calculated from my Garmin swim HR monitor. (Max HR is 180bpm)
This does give me hope that I can be a better swimmer, if I just quit sucking and actually try to swim faster.

Man... if I could achieve that bouyancy naturally.

I think most tri guys/girls w/o a swimming background don't really know how to push themselves in the pool. They tend to focus so much on getting their form exactly right that they can't go fast b/c they're scared of doing something wrong. Form is important but it will come as you learn to swim faster. I think swimming 25 yd/m sprints are a good way to get the feel of going relatively fast, then gradually extend to 50s, 75s, etc. You could do a ladder workout, e.g. 25/50/75/100/150/200, then back down in reverse order which is 1000 yards/meters total, or 1200 if you do the 200 twice. Or, do 4 x 25, 4 x 50, 4 x 75, etc, to get the feel of going faster. You should be working at least as hard, in terms of perceived effort, as you do on the B and R.

I actually strongly bet that the OP doesn't have enough muscular endurance nor strength to maintain a high swim HR for more than a few minutes, if even that.
I had the same problem in my early years of swimming. And for me, it wasn't at all a matter of not trying hard enough to hammer sets in swim practice - I went near all-out every pool practice, but it just took me 2-3 years to even build enough arm endurance and strength to keep my HR remotely up. Was pretty frustrating for me to go into Oly races, go as hard as I could go on the swim, but not even have a HR over 130 because my arms just couldn't churn hard/fast enough.
Now I have the opposite problem - I actually think I'm over-swimming effort-wise on the swim portion of the race, and I'm paying for it with a slower-than-expected run (my typical strength). Found this out recently when I did an ocean-sprint tri in frigid Pacific water with no cap and a sleeveless wetsuit (dumb I know) and ended up having to literally swim head-out of water for 3/4 of the swim since I was paralyzed if I put my head underwater with no cap for over 10 seconds. Thanks to that huge under-effort on the swim, I set my PR 5k tri run. I'm going to strike a happy medium in my next frigid ocean race (April) where I'll have full-suit, neoprene cap, but give up some swim time and hopefully claw back more of it on B/R.

Yeah, I learned that capless swim the hard way! A few guys remarked "you're brave!" to my no cap - clueless me had no idea why they'd say that. Now I know - even a thin non-neoprene cap makes a HUGE difference in cold ocean swims.
I've done similar cold ocean swims in the past, but always had a cap. I literally couldn't even move in the water without it once my head went under - it was like a pole was shoved down my head into my spine it hurt so bad!
And on the bright side - EVEN with literally doggie paddling at a slow pace (def equal to that of the BBBOP swimmers when my head was up, well over 2:00/100) as well as wasting at least 40 seconds trying to find a way to DNF the race in the first 3 minutes when I realized I couldn't stick my head under (that didn't work - we started in a boat parking area and there were unclimbable walls on all sides at the start of the swim!), I STILL managed to finish over 50%AG on the swim and still podiumed my AG in the tri simply by doing 6 strokes of head-under swimming at a time before pulling my head out of the water for 15-20 strokes. That's higher than I finished when swimming in ideal conditions in my first few years of tri!