I only swim once per week by design (and sometimes I’ll even skip a week), mainly because I’m quite satisfied with my swim results. I’m towards the pointy end of my AG out of the swim without exerting myself too much so I really don’t see much of a point in training the swim. Like @JoeX said, you’re probably not going to be gaining much by swimming once a week and if you’re happy with that, more power to you.
What I do is try to make the most of my drive time, “setup/teardown” time, and all the other small time sucks of getting myself to the pool by making my sessions around 90 minutes. I figure it take the same amount of time for all that stuff regardless if I do a 20min swim or 2 hours so I might as well put a higher percentage in the pool.
Once I’m in the water I do about 10-15 min on drills and making sure my form is on point as possible. Then for my sets I’ll usually be doing something like 400yd repeats. Once in a while I’ll do 800yd if I want to push endurance. More likely if I vary I’ll do 200yd or mix 400 and 200. Always I try to do 100yd of active recovery between my sets. So my main set might look something like:
5x
400yd RPE 8 20sec rest
100yd RPE 4 20sec rest
200yd RPE 9 20sec rest
100yd RPE 4 20sec rest
Then I try to crank out a couple fast 100yd and 50yds at the end.
Another thing that has really helped me get better quality in my workouts as well is bringing in a drink loaded with calories (I do maltodextrin and maple syrup mix). That keeps me from fatiguing too fast vs when I was doing just water.
Best of luck!