my suggestion would be do one of two things...
1 base out of a resort or air b n b rental near Williamstadt and use the dive bus or other group to take you to shore sites/boat dives. I think it would cost substantially more in total and you don't have the freedom of a rental car but still many great dives and you have one less thing to worry about regarding rental car.
2 The other would be to get a rental car and stay in Lagun or Westpunt and self cater most of your tank rentals/dives/food. Personally I think getting a rental car unless you are staying in a place with no nearby parking is a no brainer based on the cost savings of doing your own dives. I think I spent around $300 for a car for 10 days and then spent about $8-10 a tank, verses $80-120 for a 2 tank dive by boat or using the dive bus. Not disparaging either of those options, they have a lot of great reviews, I am just cheap and like the freedom to explore.
if you are just diving a few times/tanks it might make more sense to do option one, but for longer trips or more diving the second choice tends to get you more choices, better value in my experience.
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