Arm (and hand) numbness during long swims

I'm hoping "Gull" or some other MD out there can give me some advice. This being Labor Day weekend, I can't really run right over to my ortho or neuro doc. Background: I know I have minor carpal tunnel syndrome in my wrists (who doesn't now-a-days?). I'm training for an ultra long swim, and picking up the yardage. I noticed a week a go that after about 3-4000 yds/mtrs, the last two fingers of my right hand go numb, and it begins to spread up my arm, up to nearly the shoulder. I can flip over and do backstroke & shake it out, no problem, rest for a bit on the wall, and it temporarily stops, but then when I get going again, it comes right back. There is NO pain involved, and no nasty sequelae (yet), no swelling, redness, change in temperature to the limb; and once I stop swimming, it completely resolves until the next time I'm swimming a long workout. So - how to prevent it? I certainly can't just stop swimming - the "race" is 10 miles long, so I'm bound to experience this during the event, as well. I kinda wonder if it may be partially related to the water temperature, because lately we've been swimming in 76-78 degree water. Ideas?