My choice is , ash post set with wedges of cedar bedded in 5200 .
I like running it all the way down, tie the bitter end to the lower post .
Put a 1/2" bronze round stock through the post above deck. 2 round turns topped with a clove hitch and that line isn't going anywhere and wont chafe line or post. chamfer the edges of the post of course .
Under the deck a 12x12x1.5" block bored 5" square to house the post , wedged with 1/2" cedar set in 5200 . I have done about 120 or so like this with no leaks so far . I think a pure bead of 5200 1/2x1" would work as good if the post was well attached to the bulkhead.