For tank rentals in Willemstad, the Dive Bus or Atlantis Diving are semi-convenient. The Ocean Encounters shops are all on resorts so less convenient for offsite diving. Just west of town Relaxed Guides Dives is a rental/fill location along the main road. Most of the sites that require a fee are only going to charge you a usage fee if you don't rent from them. And it's a few guilders (like $4). Anywhere we rented tanks/weights, they waived the fee. Typically it's because you're using their dock/patio or other facilities.
Sites we dove - when we rented onsite it was mostly for convenience.
sort of West to East:
Playa Kalki - Go West diving. Nice facilities. Nice dock and they'll lock the locker room while you're out. You are on their/Lodge KH property to access the dive so likely need to rent from them - I would anyway.
Playa Lagun - Discover Dive - truck height dock pickup at the entrance to the beach. You can also drive right past it with your tanks. Don't sit on the plastic chairs, they're owned by the beach operator. The covered concrete picnic tables are public.
Playa Jeremi - no facilities but 5mins. from Discover Dive.
Porto Mari - Porto Mari Sports is onsite but they don't own the beach. I believe we brought tanks there.
Varsenbaai we paid/rented from Kokomo diving as it was convenient. Also we used their dock and tables.
Sunset Waters - the resort was razed as was Sunset Divers so bring yours. Drive down to the beach but park on the road - the sand is soft. Assuming the breakwater is still there, the dive is thru the break in it. Right there in 40' is the Cessna wreck. That dive is better to the east, there's a boat channel west.
Blue Bay - we didn't dive there but Diveversity is onsite. Since it's a private resort, I'd guess they charge for both.
Piscadera Bay - we didn't dive there as the reef is a swim. Caribbean Sea Sports at the Marriott or Ocean Encounters at the Hilton.
Pierbaai - bring yours or rent from the DiveBus across the street. Ask Suzy what's being seen locally currently if you do. She suggested a few good options even after knowing we weren't diving with them due to schedule conflicts. Nice people...
Directors Bay - no facilities
Tugboat - the school there is closed recently so bring your own. Park then enter the property to the left. Walk past what's left to the narrow beach. The Tugboat is off one of the larger pilings. The better dive is the wall around the point. The boat makes a good safety stop as it's 14' deep. The pier there is also a good dive if there's no ship working/tied up otherwise it's illegal.
hth, we mostly hauled tanks except we rented at Playa Kalki and Lagun. I would've probably rented at Porto Mari had I known they were there. We paid usage fees at Varsenbaai and the Tugboat since the school was still open. Also they wanted a beach usage fee at Sunset Waters but not any more...lol.
---------- Post added August 5th, 2013 at 10:15 PM ----------
P.S. if you dive the Tugboat mid-morning the 100 snorkelers from the cruise ship also make a pretty good marker. My friends were on it when they all dropped in unexpectedly. We watched -

from shore.