Well along with the above items Casper mentioned... I have assisted in finding everything from buried family valuables to ordinance removal.
I actually did make money over my life finding things both with and without a detector so yes it can be done... IF done correctly.
I started finding things for people at a very young age without a detector... I spent many years growing up on marina docks and water in general... and for a tip or agreed bucks I would find anything dropped overboard... the Beeper days were good days for me... and anchors were my best friends at 20 bucks.
Anything and everything fell in the water... mainly right there at the marina and always whilst someone was boarding or un boarding a vessel.
Expensive glasses ... keys... wallets... being the top three in which people had to get back.
I am sure if I still did this that cell phones would be on the top of that list now days.
SO... I know for a fact... IF you do a proper "flyer" and "get the word out" via self promotion... AND you are professional at what you do... you can do anything.
FOR... there is a need for... and a shortage of... "finders of things."
It can be done.