Your keel cooler is definitely going to perform in warmer water - actually it will be more efficient than in cold water (see my recent post on that subject). So 27º C is fine for cooling water and probably is not much different than your cabin air temp. Per that recent post of mine, refrigerant coming out of your compressor as a high pressure gas condenses at ~ 50ºC. A much lower cooling water temp (say < 20º C ) is actually inefficient.
Have you checked the refrigerant temps on the high and low sides of both systems for comparison and your keel cooler in and out temps? That might give you a clue, but the ambient water temperature is definitely not the problem.
Good luck with it.