Say you get 200W unused for 5 hours a day (5 hours would be generous for typical boat solar installation). That's 1kWh/day, or 30kWh/month. FPL charges about $0.10/kWh (for usage < 1000kWh/month), so you make about $3/month worth of electricity.
To put that back into the grid you'll need a grid-tied inverter. If you go the micro inverter route
(easiest for that wattage) you're looking at $150, so your payback is 5 years or so. And that doesn't count interconnection fees
, or permit fees
, or upgrades to your dock/wiring, or....
Could it be done, yes. Is it worth it? YMMV.