In fresh water lakes I get this "swimmer cold", my nose closes the night afterwards and still the next day I feel miserable. A lot of people seem to have this, must have something to do with allergy bit no one knows really what.
In the pool with chlorine I do not have this problem. I'm also fine in saltwater: just swum in vacation every day in the sea (no sharks in croatia :-) )

I race OW, but rarely train OW because of the massive 6m great whites we have patrolling out beaches. Perth is now officially shark attack capital of the world. The last 70.3 I did in may, the last wave had to be pulled out of the water by boats, jetskis etc because of a 3m shark cruising through the swim course.

^^ At 70.3 Pula (Adriatic sea) I looked around while swimming because it was so pretty and saw jelly fish here and there just below the surface. Wasn't stung, but not sure if everyone was so lucky. That'd be another reason to avoid OWS. There are advantages to living where it's a little colder. All the nasties prefer warmer climate.

Use nose clips. Problem solved. I'm the exact same way as is most of my OWS group here in the midworst. Started for all of us 4-5 years ago. All solved with clips.

I'm the one that ditched the pool for OWS recently. After 3 weeks+ of 100% OWS, I'm still at it, but I've had to pretend to ignore a few things...
1. We definitely have sharks. I think about them a lot as I stare into the abyss. Only 5 people on the planet will be killed by a shark this year. Will one be me? I buy more lotto tickets now.
2. Red Tide! I don't know if it is really a red tide, but the water is brownish-red sometimes. I haven't died or been sick yet and water reports haven't had any warnings. Just can't see anything.
3. Seals. One charged at me with his teeth showing. Those suckers swim fast.
4. Kelp. I'll be just swimming along and bam! Kelp.
5. Parking. I ride my motorcycle to OWS now for a front row parking spot.
6. Currents. Swimming nowhere as fast as you can is pretty tiring. Great workout.
7. My wetsuit is getting used a lot. It isn't as brand new as it was when I started (even though I've had it for well over a year).
8. Sand. Everywhere.
9. Fat tongue. After about 90 minutes my mouth and tongue start getting weird. I should wash my mouth out with a water bottle more regularly.
10. NO! I'm not a mermaid. I've been asked by 3 little girls if I'm a mermaid as I swim in to shore. In 35 years, nobody at the pool has ever asked me if I'm a mermaid.
I did learn a few other things. There are a lot of people doing OWS at the beach I swim at! I had no idea. I've met new people. I've also straightened out my stroke quite a bit. I had a hitch going on when I'd breath that had me out of streamline, but now that seems to be gone and I got my body roll back. I've also started bilateral breathing (after 35 years of successfully avoiding it). It just sort of happened and it feels very normal and natural now. My school swim coach would be shocked. I'm also much faster at OWS and I'm less averse to it after doing it consistently. Oh. And I'm having fun swimming! WHOA!
Oh. And I'm group swimming Wed evening at 6 pm, Sunday at 11 am, and I squeeze in a solo lunch swim, usually on Friday. Anywhere between 1,500 - 4,500 yards (but now I measure it in miles or fractions thereof).
I'm going to stick with it until a shark gets me. Hell, I ride a motorcycle to the beach to swim in the ocean with sharks and then I go home and ride my bike on the highway. It is amazing that I'm even alive. Don't tell my life insurance underwriter.