I know nothing of water melons but just to clarify, 'dead-heading' is the removal of spent flowers, so doing that to a melon would lose the fruit. I think you are meaning simply pruning?

Ah, I thought that was the term for cutting the tops off of tomato plants (and the tip, or top of anything else) to stop further upward/outward growth.

I'll stick with being literal, is cutting the tip off the vines advisable to make the plant focus on the existing fruits or will it be detrimental to the plant and existing fruits?

"is cutting the tip off the vines advisable to make the plant focus on the existing fruits or will it be detrimental to the plant and existing fruits?"

As a general rule pruning triggers the plant back into growth mode, new growth, rather than shifting plant resources toward ripening of existing fruit. This assuming weather permits. So the simple answer to your question is no, you shouldn't do it on either. But ultimately the choice is yours so experiment - do it on one and not on others and see which works best for you in your short season.

Dave

Leave the tips alone. You can pick off excess fruits though.

Thanks for the advice. Is the similar conventional wisdom to cut the tops of tomato plants in August to focus on existing fruits not effective?

" Is the similar conventional wisdom to cut the tops of tomato plants in August to focus on existing fruits not effective?"

I wouldn't call it 'conventional wisdom' but rather just the opinion of some and one that doesn't appear to be supported by much research.

Pruning green growth on any plant will trigger new green growth so focus on removing fruit that has no chance of ripening instead. Or you can always just let the plants do their own thing without any interference and use the end of season small fruits in all sorts of ways. ;-)

Dave

This is my first year growing watermelons. I am in zone 7. I
have 5 vines with 1 to 3 melons per vine ranging in sizes from a cherry to
tennis ball. The tag that came with them simply says Watermelon 5 to 30 pound.
Several Videos on YouTube says to leave 1 watermelon per vine but I assume
these guys want the 30 pound watermelon. If leave 3 per vine am I sacrificing anything
but size? I will be happier with three 10 pound watermelon that taste good vs
one 30 pounder. Is taste or fruit quality reduced if I leave the 3 per vine?

I find that for larger watermelons it is better to have 2, 25 pound fruits than 4, 15 pound fruits Yes, the fruit quality is reduced..

Does the length of the vine matter? My first watermelon is at about 3' the next at about 6' and the last at about 9'. If I cut off everything after the 3' watermelon it will have about 4 leaves. How can I have a good watermelon with just 4 leaves?
This guy on YouTube has about 50 feet of vine before the Watermelon: https://www.youtube.com/watch?v=69p7BwWgC9A

That is saying I should let the vine grow much longer before I start cutting.
The Watermelon vine has been growing quickly since July 1st. Maybe the melon at 15 feet (which is not there yet) is the one I want to keep?

I like both 7 foot melons [for early ones] and 12 foot ones.

Again, put the shears away.

I guess this guy on YouTube is wrong:
https://www.youtube.com/watch?v=kmJFhBosKwQ

Leaves are good for all plants. The more leaves you have the more photosynthesis the stronger and happier the plant. Fruit on the other side are just drains so the more you have the more drain you have unless you have a ton of leaves to support.
My best melon seems to be the one in the middle. Since this is my first try with melon this year I will just leave them alone and see what happens. I assume watermelons are like cucumbers. If it does not have the resources to support more fruit, it will not produce any female flowers.