IMO, give it land gunner mode, but make it have a slight randomization in the exact aim point.
Say, it's accurate to 200 feet. Precise enough to hit a town, or a base, not precise enough to target a hanger by anything but luck. You would need a spotter or LOS to dial it in.
Or another option would be semi manual aiming. Give it a combination of sea mode and land mode. Take the view from sea gunner mode, and clean up the firing information area. Only display range in thousands of yards and azimuth of where the gun is currently pointing, and the information for the previous salvo. All firing information would be accurate to 3 decimal places (3 feet in terms of range, and about 2.5 seconds of arc)
When you click on the map, it will give you the firing information for the point you clicked, accurate to two decimal places (30 feet and about 30 seconds of arc), in a separate box below the current one. Let's call this the "firing computer"
The player has to manually lay the gun according to what the firing computer says, and reaim for recoil.