I'm sorry to contradict getawayFK, but I much rather have the chocolate afterwards, because of stress inducing substances, and the mucus forming milk.
My suggestion would be some, not to much, pasta a few hours before with a bit of spicy vegan sauce to give you a nice steady burn inside. When your not able to have a suit, a swimming cap could be a cheap help. Warming up and doing stretching on the shore also helps to get the blood flowing and muscles prepaired. Have soft fins, because cramps are easy to get in cold water. Have a snorkle, your wet head above the water will loose more heat than submerged. Have at least 1 very capeble buddy. Learn about hyperthermia and go out before you core is cold.
Go for a couple of quality dives instead of many.
Have some heat blankets ready.
Be hydraded.
Listen very very carfully to your body, cold water invoques major effects on your body and messes up your sens for limits.
I did a couple of no suit dives in 16 celcius water,thankfully the surface was 22 C. I managed to go down to about -10m (-34ft). My pb was at the time -50m CW with suit. Going down I tried to relax, but the cold was such strong that musles tensioned and got my heart pounding like a hammer in my chest.
When I turned arround the pounding disappeared, though to me diving this way felt not safe, let allone comfortable. Personally I would advice against it, certainly without a suited and capeble buddy, good line and diver retrieval system, cause your buddies ears can lock up too!
I strongly suggest: Bite the bullet and find yourself a suit.