Yes they will be a long vine unless you got a bush type.
Watermelons are a fruit that gets better if they dont have wet feet.
They like water but they dont want to be swimming all of the time.
You will know when they are ripe when the tendrils next to the melon are dead.
This is the number one reason you cant buy a good melon at the store they are picked too soon.
The same with cantaloupes.
If you see a vine hanging from the thing it isn't worth buying.
The vine should slip or pop off the fruit when it is ripe.
Neither one will get sweeter after it is picked.
I have no idea if you can get away with a watermelon vine in the same container as a tomato plant.
More than likely so, it will just take up much more water.
Another problem will be what are you going to do with the vines?
Worth