Warming the core takes a great deal of time. I would just like to reinforce Naj's first points about not waiting even though you may not feel it. Many years ago I was diving in Lake Superior, never gets warm, beautiful sunny summer day and all was well, until signs of hypothermia started. The buddies I was diving with at first blew it off long story short I ended up in a warm shower sucking up warm drinks for a long time, before I started to feel better. Take it very seriously and cautiously and know the signs. Hypothermia while swimming would be a good thread to start, need to get going to pilot for a Tri maybe will do so when I get home. Coach Sue are you out there, hint hint on hypothermia.
Swim Silent and Be Well