i have been doing flatwater, surfing and downwinders a bit lately.
flatwater requires the paddle to be reasonably vertical to max stroke and power and keep the board straight meaning the handle hand needs to be over to about opposite shoulder during stroke. now on my short sup which has a shortboard like nose and reduced tail and overall thickness this is useless as there is no stability or board area to allow for such an un- centred action and it only gets worse in chop and wind. i find the paddle hand needs to be in line with with that hands shoulder. it creats a more centred paddling action and a more effective bracing action for corrections. precise centred paddle changeovers are also a must imo. now in open ocean paddling, i'm finding the paddle hand about centre of body is the best with a bit of each of the other two angle's benefits