Changes

In practice, for an 8-hour summer day, 40 degree latitude, the sun delivers an average [http://zebu.uoregon.edu/1998/ph162/l4.html 600 W/m²]. Currently the best commercial solar panels are ~16% efficient. At an efficiency of ~16% on a perfect sun day this is a yield of 96 W/m² averaged over 8 hours (0.768 kWh).

Now, metric horsepower, widely used in the auto industry, is defined as 0.73549875 kilowatt (kW). Assuming we need 200 horsepower (hp) to drive a 70-ft boat, this is ~147kW. At 96 W/m² this equals 1,531 m² (16,472 ft2) of panel acreage [1 m² = 10.76 ft²]. And this is just for propulsion during daylight hours. There is no extra for storage in a battery for night operations.