I agree with Demon3
The HDPE in itself is very inert, and if deep underwater, it will not be exposed to UV light and will last indefinitely.
Even if it eventually stress cracks, it will just be smaller parts of PE.
It might be damaged by marine life biting it or digging around it etc.
The additive packages might not be so friendly, but they are usually in low concentrations and are well contained, and only leach out very slowly. Food or medical grades will be better from this point of view.
Regards
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