1/2" Carbon Steel Screw Pin Anchor Shackle, Galvanized, 1-1/2 Ton Working Load Limit ( 3000 Lbs)

what "working load limit" size shackle do I need to moor my 3500 lb boat to a buoy?

Thank you for your question.
1/2" shackle to mooring your boat on the water will be fine.
WLL refers to the maximum allowed weight that a particular piece of rigging can handle under normal conditions. WLL is 1/3 of the breaking strength rating, therefore a strap with a WLL of 3000 pounds would have a breaking strength of 9,000 pounds.
Any load will be dissipate through all your mooring system (chain, rope, cleats and boat)
Note thant the weight in water is not the same as on land.
So be sure that any other part of the chain or cleats should break before the shackle.
Hope it can help you.
Thanks again