The rules as published work fine - there's no hard rule on determining which DC to use. That's why it's in the DMG, you're supposed to make a judgement call.
That said, the biggest issue with foraging in Chult is a potential lack of equipment to get potable water.
- If the characters are properly equipped, primarily rain catchers, they will have no problem getting what they need. The DC should be relatively low, possibly low enough that it's not even worth rolling.
- Cooking gear might help boil water, but getting dry wood to start a fire is tough. The availability of water isn't what drives the difficulty, the ease of getting potable water is. If they don't have a rain catcher and need to boil it, then the DC should be higher even though water itself is abundant, the materials to make it boil are not.
- No rain catcher? No cooking gear? Good luck... This is a guy bending leaves on trees to collect trickles of rain water, or finding something sufficiently clear to make a solar still. This is MacGuyver level of ingenuity, and shouldn't work every single time because then it becomes boring.
Of course, spells like Create or Destroy Water, Purify Food and Drink, and Create Food and Water can eliminate the issue entirely, but keeping them prepared and casting them daily are resources that could be otherwise allocated. (They're also only available to certain classes.) Other spells may help you make a fire to boil water, at least taking a bit of the edge off.
Tomb of Annihilation really hinges on the resource management part of old-school style gaming - Who's carrying how much of what? That said, between the set piece encounters and the random encounters, there are plenty of opportunities to separate the party from their survival gear.
...and that's when things get interesting. It's part of the challenge of the adventure. Trying to get around it isn't meant to be a matter of die rolls, it's meant to be an exercise for the players' minds.
