Considering the relative low fuel consumption of these engines, it will be one of the lowest cost item of boat ownership. Most. Boaters put in an average of between 100-150 hours/year of running time. Some years even less for us still working for a living. The rest of the time is spent sitting at the dock. If the cruising style consist of hopping from one marina to another, marina moorage alone adds up to far more than the fuel cost. Then it is regular moorage for the rest of the year, insurance, haulout and regular maintenance. The fuel cost will now be of minor concern.
If 5 gal/hrs is considered high, I suggest to narrow the search down to a single engine or a sailboat.