open water swim training?
Some form of a swim stroke dry-land drill?
to teach the next person who takes your transition space a lesson!
Both sports hate Sylvester Stalone?
How about nothing? Maybe it is this new trendy boxercise fad that has started to confuse people into thinking it is good for aerobic endurance. As a cross training workout it might be OK. As with any martial art it is hard to actually punch (or kick) with power for the whole 3 minutes of a round. So there might be some all round power/strength endurance benefits, but to get yourself to that point will require too much dedicated training. Unless you really want to fight in competition (or tournaments), what is the point?
I think some basic martial arts training might be good because of the core strength and flexibility that they tirelessly work on maintaining and developing. I’m talking about good balance and posture which helps when running and riding by using that core strength to keep the body stable while the legs and arms are freed so that their effort can be dedicated 100% to propulsion.
It’s interesting that you ask this, because I have recently started watching mixed martial arts and it has struck me that a good fighter in m.m.a. has to be multidisciplinary, which is of course the bread-and-butter of triathletes. Besides mastering the arts of boxing, wrestling/grappling, and submissions, good m.m.a fighters have to be strong and flexible. The stories on many of these guys ring similar to those of triathletes - years of dedicated training with constant focus on the weak areas. The main difference I see between m.m.a. fighters and triathletes is that triathletes generally don’t have cauliflower ears (thank god for small mercies!).