I was on swim team throughout my younger school years, and once I got to college, I did do a single season of water polo. Today, I realized that it might just have longer-term implications than me realizing that I probably wasn't cut out for collegiate athletics. Back in my water polo days, the very first thing that the coach taught the newbs to do was the egg beater kick. Simply put - if you couldn't tread water this way, you were not going to survive a game of water polo! Once we got the basic element down, we were soon doing all manner of drills, often leaving our arms out of the equation. Makes me wonder - how did Grok tread water? Something similar?
Anyway, with both arms full of 26 lb. of baby (she's 97th+ percentile), I was wondering to myself, How am I going to get any kind of exercise holding her the entire time? Then I remembered the eggbeater. Soon we were going in spurts across the pool, me eggbeating while holding her in the air. It may not be HIIT, but it sure worked muscles that weren't used to being worked!