Hi Sebrinak, and welcome. Sounds from your questions that this watermelon wine was among your first attempts. Watermelon wine is hard to make and when made it often does not taste quite as the wine maker might expect: watermelon is not rich in flavor although a ripe watermelon is rich in sugar. If you dilute the juice with water then you have a lot of water and not very much flavor.. And if you got five gallons from five melons I am guessing that there was a lot of dilution going on... But be that as it may, that won't have caused the problem of a foul smell.
Foul smells - such as rotten eggs are caused by stressed yeast. When you stress yeast by not providing the yeast with the nutrients they need to repair and build cell walls - and sugar is not nutrient - then the yeast return the favor and produce hydrogen sulfide and H2S smells like rotten eggs. You can perhaps remove the H2S by whipping lots of air into the wine or perhaps by placing a piece of very clean and sanitized copper in the fermenter for a few minutes. The copper will bind with the sulfur.
H2S if allowed to sit too long begins to form mercaptans and mercaptans are what are added to gas lines to enable you to smell a gas leak. It smells a little like skunk. Mercaptans are far more difficult to remove and for many wine makers they are one reason to toss the wine.
You say the wine tastes sweet but you don't say what the current specific gravity is. Knowing the gravity tells you how the fermentation is proceeding and if the gravity is not dropping every day then that suggests that it ain't proceeding. An active fermentation can still stall. If it has stalled then depending on when it stalled in the process will suggest whether you need to add nutrients, air or yeast. But if you need to add yeast the best way is to add the stalled wine onto active yeast and not vice versa. You want to create a starter and then when that is very active you want to double the volume of the starter with the stalled wine. When that is again very active you want to double that volume from the stalled wine and so transfer the stalled wine into the starter. This helps mitigate any systemic problems found in the stalled wine: you are diluting the stalled wine with the starter until all the wine has been transferred from the stalled bucket into the bucket with the starter.
Cloudy wine can be caused by two different processes. The first is that if there is a great deal of carbon dioxide still in your wine (and yeast convert HALF the sugar by weight into alcohol and the other half into carbon dioxide so there is very likely to be a large amount of CO2 dissolved in solution) then that gas will keep particles of fruit in suspension. Time and gravity will help the wine clear.
Another very common reason for haziness is that the fruit pectins act to diffuse the light traveling through the fermenter. It's always more challenging to remove pectins in alcohol than pectins before you have added the yeast but what most wine makers do is add a pectic enzyme (NOT pectins - you are not making jam) to break up the pectins about 12 hours before they pitch their yeast ... and with watermelon, the secret is to create a large yeast starter so that the juice is not hanging around for any time to allow spoilage organisms to begin growing in the juice. I have to admit that I do not know whether watermelon has a lot or few pectins but if there are some then that too might be a reason for cloudiness. In alcohol you would need to add much more enzyme to break up the pectins because alcohol breaks down the enzyme.
If this does not answer your concerns (and I hope that it does) others on this forum may have other explanations. Good luck.
