I did some research a couple years ago when we replaced our refrigeration. A good source for info is KollmannMarine Boat Refrigeration Specialist
This fellow has done some studies of his own. I was leaning towards a water cooled unit myself, but after reading his take on it, I went with air cooled. There was a concern that the sea water heat exchanger would spring a leak and ruin the whole system eventually. This happened in the previous Gurnert sea water cooled system that was in our boat. With the air cooled system, you do want to keep in mind where your condenser unit is mounted so it has enough fresh air to cool the refrigerant.
Where will you cruising grounds be?