You will need the following - snorkel, a good face mask, gloves, cycling helmet (cheap, unless golf balls hit you on the head while in retrieval mode) with a set of halogen lights used for night riding (AYUP is a great brand), will be needed. You may already have a sieve to scoop from bottom, then follow these steps (while submerged)-
a. use sieve to gently scoop along bottom, not too much at a time
b. bring sieve closer to lights and check contents physically
c. move hand through 'bottommuck' and retrieve any balls, place in mesh bag attached to your belt
d. also, keep any eels for pickling/putting in aspic and making this your second mini business.
This could also be done from the bank, depending on the width of the water hazard by using an extendable pole. Good luck and hope the business works.