Hi Tim,
Although I know nothing about sailing, i'm guessing your original injuring action was basically to lever your boat's weight through the balls of your feet like a fulcrum. From what you describe in the aftermath it sounds like you did some serious straining of the plantar fascia and the intrinsic foot muscles, as well as associated ligaments and calf muscles too.
Generally cramping can be for many reasons (eg, mineral deficiencies, neurological damage or irregularities, or muscle imbalance). It sounds like you're cramping because you managed to damage the muscles in the sole of your feet so much that they didn't heal properly and now they're not coping with "normal loading" and other things which put them under strain, like fins on swimming.
There are self massage and strengthening techniques that work quite well for the feet. Two of my faves:1. sit down in a chair, feet on ground. put a golf ball under your foot and roll it in circles, up and down the length of your foot, and crossways with as much pressure as is comfortable but not painful, for 5 minutes a few times a day. this will help break up scar tissue, stretch muscles and ligaments, and encourage joint mobility within the foot.
2. To help strengthen the arch muscles, sit in a chair feet on the ground. put a towel under your foot. keep your heel on the towel, but use your toes to scrunch the towel up under your foot. if it's too easy, put a book on the other end of the towel for resistance. do this for 20 seconds, 3-4 times a couple of times per day.
also for you i would be looking at stretching your achilles tendons, the front of your ankles, strengthening calves with exercise such as heel rises (single or double leg depending on current level of strength),
and also seeing a podiatrist to assess if you would benefit from some structural supports in your shoes during the day to offload the plantar fascia (ie. orthotics). and lastly, I recommend you see a sports physio who specialises in lower limbs and foot biomechanics for a full assessment and treatment plan.
good luck, hope you find some relief but it does sound like it's not going to be a quick fix (given how long you've already had issues!)
Cheers,
msk101