HDPE is very inert (unreactive). In pipe form there should be no interaction whatever with the aquatic environment. Eventually after many, many years the PE will break down due to oxidation but even then I expect no adverse affects. That's just my best guess, you would want to check for articles on it specifically.
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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PE liners are used all the time in the aquaculture industry (fish farms, shrimp farms, etc...). They are also often used for transportation of bulk liquids, including wine. I wouldn't worry too much about additive packages, but you should confirm with your supplier.
