I've kept Giant Millipedes for 13 years, none of mine have ever had the luxury of a water dish. My eldest girl died last year at an age of at least 6 and a half years and a length of 14 inches
. I've seen millipedes drink from dishes and trample mud through dishes, but I've never seen them soak in a water dish. I've always just sprayed them daily to let them drink from the sides of the enclosure, and put the food, be it fruit or veg, directly onto the substrate. The live substrate then handled the rest. In fact, the big girl used to live with African Land Snails and Hissing Cockroaches in a really manky fish-tank, and I used them to break down all the snake and lizard waste. I then broke down that enclosure after 18 months and used the substrate to seed non-bioactive substrates. I was basically using the Bugs as nature intends- as detritivores