Cool dry storage
If you leave them aboard you'll need an occasional recharge to keep them above 12.6 volts. There's little chance of freezing the battery if it's maintained above 75%. I'm not sure about the need to remove the negative lead, but it could lessen the potential of a short circuit in any part of the electrical system causing a fire.
A battery will slowly bleed it's charge if left on board. Temperature changes over the winter will allow a thin film of condensation to form on the battery case. This condensation will conduct a small trickle of current between the battery posts. Every month or two you'll need to top up the charge.
If possible, consider removing the battery and storing it at home in a dry cool spot. Never leave it on a cement floor as the cool cement will allow the same layer of condensation to form from minor air temperature changes. Even a small piece of plywood will insulate a battery from the cement.
A battery could last for five to ten years if properly maintained. Keep it charged, clean and dry. Batteries don't like to be quickly discharged or charged as excessive heat may cause internal plates to warp and touch. Prolonged cranking of a hard starting engine will shorten battery life. Charging a fully discharged battery with the engine will reduce it's life as the alternator will be hitting it with a lot of current for the full charge period.
When stored on the hard I don't need a bilge pump as I have two gabboard drain plugs. I'd rather allow the water to continually drain out than risk the high current draw of a frozen bilge pump.