30 minute swim workouts

Dan,

The Savageman 70 swim is 1.2 miles == 1,900 meters. To be properly prepared for a race of that length you need to be swimming a minimum of 2,800-3,800 meters/workout at least three, preferably four, and ideally five times a week.  Given that you said, "I'm a pretty average swimmer (about thirty minutes / 1500)," and that you have time for three 30 minute workouts plus a bit longer on the weekend, you are likely going to need to find a bit of additional time each week for swim training.

As to "I'm most interested in workouts that will improve speed, and not get too boring," true swimming speed is anaerobic, which is of little value to a triathlete.  What you really need is to improve speed at your threshold pace.  To do that you need long aerobic sets.  800 warm-up then 25 x 100 @ threshold+5/100 pace with 20 sec rest will do wonders.  Yeah, that is a boring workout but by the end of it, you will be feeling it.  You could mix that up by doing 100's one workout, 200's the next with the same total distance (so maybe 12 x 200), the same pacing and rest.  A few weeks out from the race go to 800 warm-up then two sets of 10 x 100 at threshold with 10 sec rest, 2 min rest between sets - and you will definitely want to avoid eating before those sets.  Do all of that and you will PR the swim plus come out of the water with your HR in zone 2 and the energy reserve to crush the remainder of the race.

And before you ask, yeah I didn't suggest drills or kicking.  Assuming you have half decent technique, drills are an absolute waste of time for most triathletes and triathletes at the age group level get little if any propulsion out of their kick.  To go fast, you need to swim, you need to swim a lot, and when you get done, you need to swim some more.

Just my humble two cents.

Good luck!

Scott is correct in what you should be doing in an ideal world but if you are truly limited to 30 mins, you can still get in a hard workout.

Short warmup of ~200m then something like 4 x 50 descend 1-4 to get the HR up a bit.  Follow that up with various things like 25s all out with lots of rest (say if it takes you 20s to do the 25, take 40s rest.  Make sure you aren't just thrashing in the water for those 25s all out, it should be maintaining form just at a (very) hard effort and turnover.  Another day you could do a bunch of 50s with low rest.  Lots of ways to mix it up but one thing is certain - when you get to the end of it, you should be pretty tired.  Like chest heaving, hanging over the gutter with the lifeguards slightly worried about you kind of tired.  

I've done everything from WU: 50, 100, 200, 300,..., 200, 100, 50; 5x200, 10x100, 3x300, 1000 & 4x100, and so on all based on my threshold result.  Closer to the race then I'll OWS for distance or when the Y Olympic pool goes long course I do long sets like 4 to 6x500m and +1500m.

Cool your looking at Savageman next year.  I also have that on my radar if things work out schedule wise during the summer.

I did this for awhile when I was pretty time crunched. I wouldn't say I improved my swimming any, but the swim was totally fine, and I'm sure I could have gotten by on even less had it come to that. (Getting by meaning far from PR!)

Typical things that I could do in 30minutes with fairly high yield

20 x 100

10 x 20

400s, 200s, 100s ladders faster on shorter distances

I kept the intensity fairly high with short rest as often as possible, but unless you're a pretty decent swimmer already or have a good base, it gets progressively harder over time as the lack of volume/training starts to catch up with you, to be honest.