Juice works, just make sure it isn’t already having preservatives that would prevent fermentation.
I have no experience with watermelon but have seen on other forums/ groups that people who tried to use watermelon were disappointed in the results.
Raisins aren’t going to give much in the way of nutrients but they can add nice flavor. Definitely use nutrients -honey won’t provide enough of what the yeast needs.
Yeast type and sweetness are another thing. Any wine ( or ale) yeast can be used for sweet or dry mead, but it depends on what...