As far as actual swamp-based modules are concerned... in addition to 1E's Danger at Dunwater as Mistwell mentioned above... there's also 1E's Against the Cult of the Reptile God, which has a town full of people who have become cultists of a naga in the swamp outside of the town. You can get that for $5 on DMs Guild.
But if you wanted an almost-5E swamp adventure, there's also the first section of the D&D Next module Ghosts of Dragonspear Castle you also can get on DMs Guild. Fane of the Sun Swallower is a 1st level adventure that go into a swamp to a small ruined keep with connecting cave system. Although Ghosts of Dragonspear Castle is more expensive ($18), it's a D&D Next produce so it requires a little less modification to make it 5E-ready... and the product includes four separate adventures (including FotSS) so you can run all of them as the framework of a mini-campaign as you wish.
But if you wanted an almost-5E swamp adventure, there's also the first section of the D&D Next module Ghosts of Dragonspear Castle you also can get on DMs Guild. Fane of the Sun Swallower is a 1st level adventure that go into a swamp to a small ruined keep with connecting cave system. Although Ghosts of Dragonspear Castle is more expensive ($18), it's a D&D Next produce so it requires a little less modification to make it 5E-ready... and the product includes four separate adventures (including FotSS) so you can run all of them as the framework of a mini-campaign as you wish.