Try wearing a pair of wool socks inside your footwear. I used to hike and canoe in MEC reef boots -- basically a neoprene boot with a rubber sole. I would wear a thin pair of polypro socks and a pair o wool hiking socks in them. While my feet were wet all the time, the extra insulaiton from the socks made all the difference.
Depending on how snug your ...
You can definitely train cold resistance, yes. It’s mostly a case of continuous practice.
But in your case you might find it much simpler to just move your feet more. It’s very easy to shuffle about a bit on a board.
5mm wetsuits are fine for swimming in water temperatures down to around 3 or 4 Celsius. For water around freezing, you will want a dry suit if you plan to spend much time immersed.
Be aware that there are no suits suitable for those temperatures that are easy to swim in - the shoulder joints are stiffer than you would want.
Of greatest criticality in cold ...