Automated ma deuces sound like they’d be awesome to witness. You’d need low light rather than thermal since zombies probably emerge the water at around ocean temperature.

Can it be calibrated to shoot high? Unless it’s scoring headshots, it’d have to tear the zombie apart to get it to stop, which could be ammo spendy. If spare manpower is available, having a shift guard in a tower with a reasonable caliber bolt-action (Mosin Nagant?) would be a lot cheaper (.50 BMG is what, $3+ a round commercially?)