Hi Raymond,
I looked into this a few years ago. I take it you have no mains/shore power to use a conventional charger, nor a method of charging it up when under way. I would look to link it to your engine alternator if at all possible, so it's getting charge whenever the engine is running.
The marines turbines/generators are expensive, and only deliver a few amps. The good news is they can keep delivering it 24 hours per day, so need carefully regulating. The vibration and noise can be significant, and wherever they're placed they will need careful bracing on a pole with supporting wires. This makes it difficult to remove if it's in the way. While it will be more than enough to keep a leisure battery on a boat charged, you might have other options open to you.
Have a look on Google (or similar) for "marine wind generators" for costs, performance etc.
If you then look for "marine solar panels" that might give you another option, which could save you a thousand pounds or so...I have a small £40 solar panel that I use to keep my car battery charged when away a couple of years ago, and it fits inside the back window. The car started first time after 16 weeks, despite the security system being active all of that time.
A third option would be a portable power pack you can charge at home, or even remove and charge the leisure battery itself at home.
Others may have a different solution, but good luck, whatever you decide.
Mike