Every few hundred feet, there should be a small cluster of lockers with code-operated electronic locks. Beachgoers who want to go in the water, but aren't crazy/stupid enough to leave their cell phone, PDA, wallet, etc. unguarded would call the phone number listed on the lockers (using their cell phone)
and use a credit card to purchase one or more usage credits.
Credit card processing fees are non-trivial, so the rate for a single usage credit would be fairly steep (say, $5), but quantity discounts would rapidly kick in (say, 5 credits for $10, 15 for $25, 40 for $50... expiring 365 days from purchase if not used). To cut costs, purchases would be entirely automated... the userID would be the user's phone number, the pin code would be user-selected.
The moment a credit is used, a clock begins silently ticking that's good for unlimited locker use for the next 6 hours. Every 6 hours, another credit is consumed until they're exhausted. Disposition of forgotten/abandoned property would be handled in a fairly straightforward manner: at the time they purchase their usage credits, users would be asked to choose how they want to have forgotten property handled:
* $50 charge to user's credit card to FedEx the property to the credit card's billing address (or ship to foreign country via airmail)
* $30 charge to the user's credit card to ship the property to the credit card's US/Canadian billing address via normal surface mail.
* $20 charge to hold the property for up to 72 hours. The cell phone used to purchase the credits would be called automatically every 24 hours as a warning (won't do much good if the phone itself is in the locker, but it's still a nice gesture). If no contact is made with the customer, the property would be disposed of as the company sees fit. Otherwise, the customer can apply $10 of the fee to the first two options to have the goods shipped.
* finally, a customer who declined to choose an option at purchase time, but remembered to call before the goods were disposed of could choose one of the options with an additional $20 penalty (remember, the $50/30/20 is NEVER charged unless they actually do forget their stuff. It's not a scheme in and of itself to reap a windfall -- it's a courtesy).
Basically, there'd be an employee who visited each cluster at least once per day to bag, identify, and collect forgotten items. Since he/she'd have all day to make basically one visit to them all, two or three employees would be more than enough to service an area the size of South Beach, with maybe two or three more if they also serviced Crandon Park (Key Biscayne) and the rest of Miami Beach.
Physically, the lockers would be the approximate size of a post office box (say, 4 x 6 x 12 inches, with maybe a few more that were large enough to store larger things for double or triple the credits). They'd be powered by rechargable lead-acid batteries (read: car-type batteries, maybe with solar cells if vandalism didn't make them cost more than just physically swapping the batteries with new ones every few days for offsite recharging). Communication with each cluster would be one-way, via the repurposed service of some local paging network or other commercial radio service (a few times per day, a list of customer IDs, hashed PIN codes, and remaining credits would be broadcast... and of course, newly-purchased credits would be broadcast immediately). Credit usage would be counted by having the employees download the previous day's transactions into a portable data collection terminal (read: PDA on steroids) during their daily visit. Why so low-tech? For maximum convenience, the lockers would be just past the high tide line, mounted a few feet high on posts (so you could stash your stuff on your way to the water, and pick it up on your way back to your towel and stuff.) Running power lines underground (the only acceptable way) to hundreds of little clusters would just be cost-prohibitive compared to the cost of swapping a rechargable battery every few days (they wouldn't need much power... just enough to power the embedded computer and an occasional brief burst to shift the solenoids left or right to lock and unlock the doors). Why electronic locks? In the long run, it's cheaper than dealing with lost key replacement and completely automates payment processing (as opposed to, say, having the employees collect pails and pails full of quarters, trying to figure out which lockers have been occupied for "too long", and leaves no user-friendly strategy for dealing with forgotten goods.