The answer to this question is highly relative to the intensity of the sunshine, the size of the solar panel, quantity of associated batteries and frequency of use.  If you only use your boat lift once or twice a day then you should always have power regardless of the size of panel and weather.  If you plan on using your lift many times over a short period of days or hours then it is recommended to purchase a larger panel and perhaps a second battery.  This ensures you that regardless of weather and frequency of use that you always have power for your lift.  The second battery also has the added bonus of speeding up the motors ability to go up and down.