I bought huge amount of supplies like water, rum, rice, fish (500 in every stack) but I'm getting constant information about dehydrated and starving crew because of no food/water supplies. All items are in stash, can't be split into lesser stacks. I didn't have this issue with other Watcher during exploration even with 500 items of all types.

Did you move supplies into ship’s consumption que (ship management, the most upper right inventory box). Crew won’t eat supplies from your general stash.

 

I have reported an issue I encountered with splitting stacks of item in ship management view - not sure if it’s the same problem you have. Or something completely different:

https://forums.obsidian.net/topic/96080-problem-with-splitting-stacks-of-items-in-ship-management-view/