Ironman is a long day, and intensities are actually quite low compared to short fast efforts, or even shorter sprint races. Hence in reality not much of a warm up is needed. Especially if a cold day, with cold water temperatures, and then standing around wet and really cold before the race starts.
A short swim if possible (not always), with your approach ideal – better than a continuos swim, as often space is limited to a small roped off area anyway. No need to swim longer than 25m, just do 5 – 10 minutes of 25s, with every 4th one a little quicker than the others.
No need to run, that just gives heavy legs in the swim – the part of the race most of us have the greatest challenge with anyway.
Hope that helps.