I find the first few minutes of the swim are the hardest, as your body is adjusting to the cold, surface blood vessels are constricting, etc. The next 5-15 minutes are fantastic! I don't usually push it much beyond that.
But any discussion of the benefits of cold water swimming also needs to highlight the risk. If you're thinking of trying it out, make sure you're acutely aware of the temperatures you're dealing with, how long you're planning on being in the water, and that you understand the body's reflexes to cold water submersion. Here's a good resource covering these points: http://seagrant.umn.edu/coastal_communities/hypothermia
I've also noticed that there are a few phases you go through. First there's the initial shock--that lasts a few minutes as you say. Then you adapt and it starts feeling good. But if you stay in for long enough, you can start feeling cold again in a deeper more full-body way that indicates your overall body temperature is decreasing. That's when you want to get out :)
I now live too far from the sea to do it regularly but I was just up in Cape Cod at the weekend and took a dip in the ~40F water. This was on the colder end of what I would ever do back home and I wasn't accustomed to it, so it was a little difficult but I certainly felt very refreshed after! I still usually do, as others have commented, turn most of my morning showers to cold at the end, if not taking them entirely cold, but it's just not the same... The saltiness of seawater in particular seems to be very therapeutic and might be an additional factor.
Yes I think the saltiness plays an important role, especially the magnesium. It’s similar to taking a bath with epsom salts.
Where I live now it's 58 in the summer, 53 in the winter. After going in all summer I couldn't wait to see what 53 does to you.
I'm the only person out there in a speedo, everybody else is in a body suit. Beach walkers are bundled up. I'd like to know the effects on brown fat.
First time I did it I felt stoned afterwards. I don't really get that feeling anymore but I definitely feel better. My cold tolerance is high throughout the week.
At first the water is too cold for my feet but over the course of 20 minutes I don't feel anything at all. Helps that I'm over 25% body fat. I finally get up to my neck. It's still difficult to repeatedly put my head underwater because you get something akin to brain freeze.
The scientific benefits of full-body sun + salt water absorption through skin + cold body exposure are not popular, even if the science has been done. My common sense tells me it's great for me.