Electric water heater? The first thing I would do would be to replace that heater with gas or propane. It's going to use up almost all your generator capacity.
10 circuits? Each 240 circuit takes up two circuit breaker slots. So well and water heater use two each; central air would take another two. If you already have separate main disconnect and main lug panels, replacing the main disconnect with a transfer switch as suggested by B&D is the simplest and best way to go.