Ordinary milk is not needed in a Maine Coon's diet and can cause digestive upset in cats that do not tolerate lactose well. Fresh water is the appropriate everyday drink. Products sold as cat milk are different from cow's milk, but they still should not replace water or a balanced main diet. Check the label and account for any calories if your vet considers such a treat suitable. Do not use milk to encourage an unwell cat to drink without asking for advice, and do not assume a large cat can tolerate a larger amount.
If your cat develops diarrhoea or vomiting after consuming something unusual, tell your vet what it had and how much. For kittens not yet weaned, feeding needs are specialised and should be discussed with a veterinary professional rather than improvised from household dairy products. If offering a labelled cat-milk treat, measure it rather than repeatedly topping up a bowl, so the amount consumed remains clear.
For product comparisons alongside this advice, see Kitty Buffet and Purr-Celain bowl. The details below explain their sizes and uses.
Further reading: cats.org.uk; bva.co.uk; battersea.org.uk; cats.org.uk; pdsa.org.uk; pmc.ncbi.nlm.nih.gov; pdsa.org.uk; cats.org.uk.
Kitty Buffet
Purr-Celain bowl
2.5 L stainless-steel water fountain