When you state aquarium sand is expensive for this type of project that has me worried. Have you realized how much just food will cost daily for a setup this size?
I wouldn't think pulling water from the ocean would be a good idea, especially in a residential area. Way to many variables... Neighbors dumping stuff in the water, boat leaking fuel/oil, algae bloom in the inlet where you are pulling water, maintaining salinity, maintaining proper temp
Doinga concrete style pool it would be difficult to do one even only at 30-40k galling for less then $75k. At least that's what the going price is here in Ohio, neighbor was looking at one and it was $65k 7 years ago and under 30k gallons.
I do agree a pool aquarium would be really cool and has been done many times with people with koi. I think you would be better off doing a more manageable size, possibly something that could be made larger in the future and use 100% synthetic water unless you can pull ocean water from offshore, far enough away where the salinity is consistent and contaminants are a minimal risk.
You can buy salt in bulk, save a LOT of money that bay buying skids at a time or a semi full of it With the money invested in fish to me this is the only option.