One little trick that I would recommend would be to use paper towels to blot off any extra moisture from the watermelon before spreading on the whipped cream. The whipped cream might want to slide off if the watermelon is very wet. I made this and stuck it in the fridge and it held up very well over night. This was because I used heavy whipping cream instead of just regular whipping cream. The heavy whipping cream is more stable and stays whipped much better. I assume this is because it has a high fat content but I’m not actually sure. But now you know. If you want your cream to stay nice and whipped, choose heavy whipping cream. If you are going to be whipping it and using it right away (if you were making strawberry shortcake or something), then regular whipping cream works great.