I am also a diabetic and watermelon is one of my favorite fruits to eat. Just like anything that involves carb's or sugar you have to eat it in moderation! The best thing I can say to you is to eat the watermelon and test your blood glucose after you eat it. If it causes your blood glucose to raise lower the amount next time. You have to find what works best for you. Enjoy your watermelon and remember moderation is the key.
Fruit is better for you than fruit-juice because it still has fibre - which slows the assimilation of its sugar content. That said, watermelon is fairly high in sugar - so the quantity should be limited. You should also think about what else you can eat with it at the same meal. Some other foods will help slow the absorption and help even out your blood-sugar levels.
Watermelon contains Lycopene (also found in tomatoes) - which is an important nutrient - especially to prevent prostate and other cancers.
I don't think watermelon by itself would be the best fruit to choose for a snack. A small apple or pear would be better if that is all you are having.
A good way to enjoy watermelon is to include it in a small home-made (no added sugar) fruit salad which contains apple, pear, and other fruits.
As previously suggested, take your blood-sugar readings to see what effects watermelon has on you.
Is watermelon good for a diabetes snack.