Garmin Swim Manual Lap Counter

There isn't a manual lap counter. I can think of a couple of options:

1) Set the pool length to 20 m, use the watch as usual, and divide your total distance by two. I can't guarantee this will work though. One reason there is a minimum pool length is that the watch needs to see at least 5 or 6 strokes between turns; also the turns can't be too close together in time. So, whether this trick would work for you will depend on how fast you are and how many strokes / length. (Wear the watch on the arm you tend to stroke with first off the wall.)

2) Pressing Pause/Unpause creates an interval (with a short rest in between). The watch does keep track of how many intervals you've done so you could Pause/Unpause every, say, 4 lengths and then your total lengths will be the number of intervals * 4. I guess this is most similar to the manual lap counter you are looking for.

I hope that is useful.

That is useful - thanks.

It might actually be possible to swim laps diagonally in the family pool - I'd have to calculate the distance, but that might get me up to the minimum supported. My technique at this point is so bad that having a square, flat wall to kick off of doesn't do me any good anyway

A good turn is important to help the watch accurately count lengths. Your turn technique (what you do at the wall) doesn't much matter, but a strong push-off followed by a glide phase is required.