The same basic rules apply to pretty much all alternate energy projects:
How practical are solar powered boats?
You have to be hard-nosed and realistic about:
1) how much energy is really available
2) evaluating your energy requirements
3) understanding how well the two match up.
The latest edition of the Solartanic uses two 24 volt, 54 pound thrust Minnkota trolling motors.
1) How much energy is available?
2) What are the energy requirements?
I found my solar panel by cruising the web. The going rate for new PV (Photo-Voltaic) panels seems to be a bit under $4 a watt. I wound up buying a 70 watt panel for $269.00. This can produce a maximum of 70 watts IF full, unobstructed sunlight is shining at a right angle onto the panel. Note:
Directly powering the motors from solar panels is an expensive and inconvenient proposition:
3) How well do power requirements and availablity match?
After initial investigation, I quickly disregarded the idea of mounting the panels to the boat. It just didn't seem practical: it was too expensive and certainly didn't fit the "look" I was after. I've seen boats on the web that do run directly from solar panels, but they tend to have large flat surfaces for mounting them. Catamarans are often popular for this type of boat since they provide a lot of square footage and can provide a somewhat higher cruising speed at low power.

The obvious alternative is to run on batteries and mount the solar panels to the dock instead of the boat. It is (usually) much easier to orient the panels to maximize the energy collected. Since I live right on Lake Boon, my boat, like many pleasure boats, will spend most of its time at the dock where the panels can charge the batteries. My estimate on how I typically use my boat:
I picked 10 hours a week as an average utilization.
Two 70 watt panels (in series to provide 24 volts) can provide a maximum of 140 watts. Making some allowance for less than perfect efficiency, it takes 4 or 5 hours of good sunshine for each hour of cruising at 480 watts, or between 40 and 50 hours a week. Given the long days of summer, an average week should provide enough sunshine to support 10 hours of cruising. My experience at the end of last summer (with the Solartanic's original 12 volt motor and single solar panel) seemed to confirm this estimate. I've recently received my 2nd solar panel and will be observing the new setup as soon as I get it installed.
Is it worth doing this? The panels cost about $540 and will be generating about 50 cents of electricity per week. However it would cost several hundred dollars to have an electrician install a GFI protected line down to the dock so I decided the solar panels were worth it. So the short story is:
Analyzing a specific design case helps illuminate some of the issues with AE. The Solartanic can serve as a useful example. I had already decided on electric power for my boat. Electric boats running on storage batteries have a history over 100 years long and continue to be successful in niche markets where peaceful, sedate cruising rather than speed is the priority (water skiing will have to wait for higher density storage). The question was: do I run an AC line down from the house to the dock to allow convenient recharging of batteries or can I do it with solar panels?
Summary: My two 70 watt solar panels, along with a controller and wiring cost about $700. This will support about 2/3 of a horse power for 10 hours a week during the long days  of summer. I decided this met my requirements so I installed the panels on my dock. But people expecting a couple of solar panels and 4 storage batteries to be a replacement for a 200 horse power outboard motor will be greatly disappointed.