I would go for what we called a camper shell over just the bed top for diving.
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Full sized van, you can get kitted up in there if need be.
Pickup would be my last dive vehicle choice, even with a cap.
Bonus! if you go super budget, you get that kidnapper look! Imagine their face when they think your a creeper and then you pop out in scuba gear.
Full size vans (with a V8) are still man eligable.But then you have to turn in your man card for driving a van.
This is the key gender difference herewith a V8
Mildly surprised that fuel consumption didn't make it to your top ten requirements.
There's a lot of difference between running costs of most of the vehicles in that category.