What about parking near the bridge? If I rent a car I might as well just stay somewhere off the island and drive.
A car is the way to go if you want value for money.
East of the bridge is a high rent area (singer island) & we are coming into the high season. West of the bridge is sketchy & I don't recall seeing hotels in that area that I would recommend to anyone who can afford to be elsewhere, not even a tough New Yorker.
Visibility at the bridge has not been good since the last hurricane came through, at least not as of Tuesday, which was the last time I was able to get up that way. I'm expecting things clean up in the next few weeks, but time will tell. By the time February rolls around, chances are very good that conditions should be fully back to normal. We are at the tail end of hurricane season now.
I only dive there at or near high tide. The bridge is very close to Palm Beach Inlet (ALA Lake Worth Inlet). When clean water washes in from the ocean. visibility becomes good. 30-40 feet is not unusual. Sometimes it's even better. When the tide changes & water washes down from the ICW, you usually get mucky water that is often not so good to swim in, let alone dive in. When rains have been heavy there is usually even more muck in the ICW. Since the last hurricane, the ocean water has also been turbid. That's the problem right now. No clean water has been washing in.
I agree about getting a local guide for your first dive. Either dive shop should be able to rent one out to you. The people who post here frequently are probably also good choices, if they are willing. I had planned to get scuba_jenny to guide me when the water quality improves. Even though I have dove there many times, I expect that I can still benefit from her guiding me because she seems to find a lot of things that I miss & she seems able to identify a lot of things that I don't know. ...and she is often willing to guide other divers.
If you are serious about sleeping in the back seat of your car, you might be able to get away with that in the parking lot of a Walmart or some place like that. Personally, I'd google up a chain hotel that is 20-30 minutes inland & try to find a reasonably priced room. Once you are west of I-95 by a mile or two, prices start to drop. Once you are west of the Turnpike, they often drop some more, unless you end up in some place like Wellington, which gets expensive due to the Polo people out there. If you want to take a longer ride & stay out in some place like Pahokee, it should be dirt cheap, but that's like an hour drive each way & not the nicest of towns. There's a Studio 6 in West Palm Beach, near PBI airport that shows up on Kayak's website for less than $100/night. That's probably about a 25 minute ride to the bridge & maybe 5 minutes to the airport. It's not a downtown night life spot, but it's also not a hell hole area. You could always try something like air BnB.