I already mentioned it for the Garmin sync, but it would work for the Google Fit sync as well. Google Fit also stores hydration data: Nutrition data types | Google Fit | Google Developers. Maybe it would be fun to sync that to intervals as well.
Like most sources it would require a solution for a simple mapping problem. The hydration in the wellness data is a subjective 1-4 scale while most sources would provide a volumetric unit. I see two solutions. Either an additional field where a volumetric measurement could be stored or a mapping in settings where you would define what you would count as Great, Good, OK, or poor hydration.
The same holds for the nutrition data that could be stored in Google fit. This one would be even easier where it would be just the sum of all food items.