With battery technology getting more and more costly and the prices of even "inexpensive" Wal*Mart batteries climbing over $100.00 each keeping your banks healthy can be a good return on your investment.
Installing a panel in the 30 to 85+ watt range can be cost effective and it can lead to longer battery bank life.
With dockside corrosion issues running rampant in many marinas solar dock side charging is gaining in popularity. This allows owners to "unplug" their boats in thus avoid many potential corrosion issues.
Some marinas charge as much as $18.00 per foot per season for just 30A shore power. That expense for a 40 footer will run you $720.00. Granted, if down South, you'd lose the ability for AC but the savings in electricity charges alone can pay for a very nice array. For boaters who don't need dock side refrigeration or air conditioning this can mean future cost reductions beyond year one. With a large enough array many boat owners are able to fully power DC refrigeration needs while off cruising or while dock side.
Quality panels these days are running under $2.00 per watt making them a significantly better value than they have been at any time in the previous 25 years..