Do you only get nausea swimming crawl? Does it stop if you switch to another style? Have you had a coach watch you swim?
I can only but guess, but I bet that you're moving your head around a lot when you swim, you may not notice this. This can easily happen when you swim crawl and I suppose cause nausea. A number of things may contribute or make it worse:
- your head roll with the body with each stroke
- high stroke rate making the movement faster
- breathing _every_ stroke
- crossing over in the stroke so you roll too much
- lack of fixing on a point
- dehydration and exhaustion
(this one probably doesn't apply
You should dissociate the body roll keeping the head still, we loose mobility as we age, you may need to do some exercises for the neck. Get a good rhythm in your stroke, if it's very fast it will make things worse. I assume you don't, but some do breathe with every stroke when they first start out learning crawl, that means a lot of head turning. If you cross over, that is your hand crosses the long axis during the stroke then it will lead to excessive body roll. The best thing you can do is to get a coach to watch you swim.
In the pool follow the black line, ok, if you're sharing the lane you need to stay to the side of the black line, but fixing your eyes on it may help the motion sickness. For the same reason motion sickness, dizziness and nausea is worse in OWS, there is no black line. If you watch you'll see lot's of people struggling to regain balance when they get out of the water.