Swamps are the location for a lot of horror stories and RPG adventures, and they often come populated with zombies. Of course, once adventurers gain enough experience they begin fearing zombies a lot less, even in relatively huge numbers. But it seems to me that too rarely are the possibilities and implications of fighting undead in a swamp really used to their full potential...
Imagine tough adventurers with swamp water up to their waist, fighting off zombies. What if the zombies, instead of trying to bite, claw or swing crudely with their weapon, were instructed instead by their creator to form groups and grapple adventurers, dragging them under the swamp water where they can't breathe (Something the undead don't need to do) and holding them there? If an adventurer is dragged down into the water with his torch or lantern, it would also mean less illumination for the living. And sure, one zombie might have trouble grappling an energetic adventurer, but five zombies grappling at the same time?
And since we're at it, the necromancer could instruct some others to remain beneath the muddy water level, crawling to attack adventurers' legs from where they can't see them.
One imagines the adventurers would probably arrive on a raft first, but the zombies instructed to remain underwater could easily be instructed to tip said rafts over too. Zombies and other undead, at least in d20-based games, don't actually require their eyes to see. It stands to reason that the muddy water wouldn't particularly blind them.
Don't forget that open wounds in swamp water are very likely to infect. Wounded adventurers might be looking at a Filth Fever or other diseases wearing them down in the days to come, should they survive this. Even if you're using the traditional D&D zombies that do not transform people into zombies from their bite, being bitten or clawed by a rotting thing should still carry the potential for more typical but still debilitating diseases.
Oh and blood in swamp water attracts other things, things which love to feed on warm bodies and are likely to be uninterested in the zombies.
Giant Leeches maybe? (OSR stats for leeches HERE!)
Or a good old alligator?
Another interesting thing is that in some games, zombies are resistant to bashing weapons while undead skeletons are quite the opposite, being resistant to cutting and piercing weapons. Experienced adventurers might have different types of weapons to best deal with the type of undead they'll encounter, zombie or skeleton. I recommend you mix it up, have skeletons along with the zombies. Unless an adventurer can fight with two weapons, he will find himself without the single perfect weapon for the occasion. He'll have to either use a weapon that properly harms only half of the enemies, attack with his untrained off-hand too or waste time switching weapons, all very inconvenient.
Finally, don't have the Necromancer who created these undead pull back to wait for them at a later encounter so the group Cleric or Paladin can Turn Undead and get rid of them quickly; no, have the Necromancer stay back but still in the encounter, perhaps up in a tree to be safe from giant leeches and alligators, bolstering his undead minions so they'll be less easily dispatched.
Go ahead, go fight zombies in a swamp now.