with that kind of time, i think you have two main options:
-30-minute straight swim (not all the time, but occasionally, as a fitness test)
-30 minutes of very, very high quality drills and sprints
i’m assuming when you say 30 minutes that you’ve got 30 minutes in the water - not 30 minutes to get to the pool, workout, and get back! i used to swim on my lunch breaks and would only do quality - if i wanted to do longer pace sets, straight swims, kicking, stuff like that, i’d save it for the weekend.
-mike