Italy Food and Souvenirs

Venice:
One day vaporetto (waterbus) pass is €25 per person (recommended since otherwise it's €9.50 per ride for non residents)
Restaurants in Venice are more expensive than other Italian cities, but a sit down restaurant should not cost more than €40-45 per person on average, unless you go to super expensive restaurants. Street food (like sandwiches) of course would be much less but I presume you'll like to sit down at restaurants at least for dinner.
Souvenirs are up to you. I rarely buy any. They are just dust collectors.

Naples:
much cheaper than Venice. You can easily eat at a sit down restaurant (especially pizzeria) and spend less than €30 per person. Street food much cheaper of course. Pompeii can be reached with the Circumvesuviana commuter train for under €4.
The admission ticket at the Pompeii archeological site is €18. The National Archeological Museum of Naples (MANN) downtown Naples is €22 and is valid for two consecutive days.

I don't know much about the cruise. That I imagine would vary greatly depending on company, number of days, accommodations, etc.

I hope you don't plan to budget $500 for all of the above, including the cruise. Unless you are a joker intending to pull our leg

Budgetting for "a day in Naples" is impossible to answer without knowing what you plan to do. Hop-on-hop-off bus? Museums? I have no idea what kind of food you're looking at -- small corner cafe? bakery? sit down restaurant? Only you know what you want to spend on souvenirs.

Do you mean $500 total for your three days in Venice, Naples, and Pompeii? How will you travel between these places, and is that included in your $500?

This doesn't include cost for the cruise at all or anything like that haha! Mainly food for the day in Naples and Venice and the transportation from hotel to tour sites and then souvenirs.

How many people?
What sort of souvenirs?

2 people as far as souvenirs I’m not 100% sure but items that I can hang up on my wall, paintings we have a room that we hang up all our travel items in. Something small obviously in order to get the back home without it being a hassle or stressful.