I just swim in pool or the ocean. If you don’t have access to an open pool just use time. For instance, swim 45 min to an hour. 45 minutes can give you a very good workout.

Unfortunately, time and effort seems to be the only way to estimate distance. I swam 2.4 miles once (virtually) tethered to the wall of my backyard pool that way. The funny part was how it made me walk into walls and nearly fall down several times when I got out because the chop I was making set me up for "sea legs". That was much more like rough open water than I expected, since I had no lane ropes to cut down the waves I was making for myself.

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I think if you're really trying to track TSS or something like that you have a couple options:
1. use the tri or swim version of garmin's HR strap, you can get hrTSS from that
2. if you know how many strokes you take per length, count your strokes. Use the drill mode to enter the distance every time you take a break.
3. if you can put a timer at the bottom of the pool, swim for whatever your estimated 100m pace is at a certain effort, then enter it using drill mode.
4. just estimate based on your time and effort

I also have a 935 and use the drill mode to correct it when it doesn't read the right number of lengths (happens pretty often in a short pool or when going really fast)

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Using your Garmin 935, take a look at one or several of your prior swim sets in a pool and look at your workout on Garmin Connect and on the second icon that looks like a page, it will list your total strokes, avg stroke rate per minute and maximum stroke rate per minute. If using the open water option, the pace and speed with be way off because it cannot measure distance nor will the pool swim option provide you with any accuracy. But, you can take your stroke per minute and total strokes and the time and get a pretty good estimate of pace for the swim. Just look at some of your prior swim paces and stroke rate and it should be close. I do this in my endless pool.

Just curious - how would you compare the sensations and even the training effect of swimming in your endless pool compared to a tether in a fixed pool?

It really depends on the pace. The endless pool has an app that you can use and you set your pace for the current per 100/m or 100/yds. I would have to say at the higher pace per 100 it definitely has a feeling of swimming against a current vs. static water in a lap pool. But, it definitely will make you faster as a result as you increase the current speed. I will set it at an easy pace to warm up and then turn it up as I do intervals. I can definitely feel the gains when I swim open water or in the pool. The app also tells you distance based on pace and time. So if you have it set at 2:00 per 100/m, it will start adding meters as you progress, so after 20 minutes I will have swam 1000 meters, etc. You can also set the duration and program in workouts so the pace adjusts automatically and will shut down automatically once you've hit the duration you set. Hope that answers your question.