Even as a kid many years ago, I sometimes get acute cramps in my calves windsurfing especially when waterstarting (the rear leg). Its the only time in any activity I do that does this. Eg no problems surfing or kiting. No difference whether with or without wetsuit. I drink alot of water. Any idea what is wrong with me ? Seems to come more often now with age. I usually shake it off but it is starting to bother me now.

Do you stretch your calves before and after your session?

i think thats normal. kitsurfing has a different stance which uses quads/core and is much more efficient.
winsurfing uses calves much more. If you get a cramp hold onto your foot and dorsiflex the ankle hard.
And stretch before you windsurf.

I take 1 Blackmores bio magnesium tablet a day to prevent cramps . It might help?
Do you see a physio/ chiro ? I see a chiro regularly for prevention. He doesn't manipulate much but works on muscles and general body alignment. He gives maintenance exercises.

I used to have a very low salt intake, my cramps have reduced since upping my salt intake slightly.
Magnesium also helps.

Relatable, used to have the same when starting out -at the time animals were still talking- (rear leg calve cramp when waterstarting).
In my opinion it's about learning to actively relax (thinking about that leg) during that phase. Subconsciously one can put a serious strain/contraction/arching on the foot/leg causing the calve to cramp on itself, whilst in reality the rear leg has not that much to do to get onboard :)
Problem with cramping is it almost reinforces itself into muscle memory, so stretching is certainly not a bad idea. Might take a while to remove.
Supplement classic (if needed) is magnesium+calcium

Thank you all. Will keep you posted if I find a way to calm it down. Its just odd that its only on that waterstart moment, when lifting the knee and before I put effort through the leg. Its fine if I leave the back leg flat in the board. Waterstarting strapless kitesurf is not too different in beginning position, and no worries there.