> You can also float in seawater in the fetal position. Your head will be below water but it is easy to come up for air periodically.

Easy to periodically come up, like, every 30 seconds? 3240 times in a row?

Well if you stay calm its more like once a minute or two. When I learned the technique they forced us to stay in the water like this for half an hour, it does kind of suck after a while.

How much heat would you lose by submerging your head and neck, versus how much would you gain by keeping the fetal position? I'm not sure if that's a positive tradeoff.

Keep in mind fetal position requires almost no effort to maintain.