...Washington Slagbaai National Park....Most of the big-name rental places will not let you take their vehicles there.
This is important. I have seen contract wording which would prohibit diving as well, including pickups(!) from some of the big-name companies. Read the fine print!
I'm not sure AB's policy but I thought I'd been told they don't allow their truck to go there.
AB allows their trucks to go there, not their SUVs/cars. All their cars are Chinese. Their pickups are Great Wall Motors Wingle 5. (I know...). Not too fancy, but get the job done just fine.
I have done some diving up in the park and personally, I don't feel there are any dive sites that I would go out of my way to make a day trip to dive up there (if that were the sole reason for going).
Same. Essentially to cross them off the list, but not enough to warrant the daytrip unless you already planned to visit the park.
If you continue past Karpata and go up and around BOPEC and back down to the ocean, there are a number of marked and unmarked dive sites all the way to Playa Frans, which is the last dive site. Candy Land is the first dive site you'll come to and one of my favorites. It is unmarked, except for a huge pile of stacked up rocks that indicates its location. Nukove is also a nice dive, but on occasion can be riddled with mosquitoes, if so, get in and out quickly.
Candy Land, Taylor Made and Carel's Vision are my three favourite sites on Bonaire. All are reasonably doable from shore. Nukove and Playa Frans didn't do anything for me.