Dragonnes become Huge at 13HD. I don't think these should be as low on HD as giant animals or vermin, either. So I'd be willing to go as low as 13HD, even though I'd prefer higher.
Higher Dex is ok with me.
Let's start 'em at 18 HD.
For our comparison dragons, we have:
Abyssal Drake: 10 HD, +12 natural
Hill Landwyrm: 20 HD, +15 natural
Jungle Landwyrm: 28 HD, +15 natural
Tundra Landwyrm: 24 HD, +18 natural
So maybe +14 natural?
The water temperature is strongly affected by a magical secretion from the swamp wyrms’ scales. Guivre pools are always steaming hot, melting snow and ice around them like hot springs, while knucker holes are chill, fog-shrouded, and sometimes iced over, killing all vegetation around them. Stagnant waters from these lakes are poisonous to all except swamp wyrms and other reptiles, thanks to the secretion; those drinking the water must save vs. poison or suffer 1-4 hp damage per round for 1-6 rounds.
I'm fine with Int 6.
Breath weapon looks good, but let's drop the "up to 3/day" like we've done for most creatures with breath weapons, allowing the standard 1d4 rounds to suffice.
I think we should writeup the secretions similar to the fire elemental's burn ability (minus the chance for combustion), and note that underwater the burn works to a x-foot radius.
A swamp wyrm will also take only half damage from attack forms similar to its breath weapon (i.e., fire and heat for the guivre, and frost and cold for the knucker).
Stagnant waters from these lakes are poisonous to all except swamp wyrms and other reptiles, thanks to the secretion; those drinking the water must save vs. poison or suffer 1-4 hp damage per round for 1-6 rounds.