I think I see what happened here. The post you're calling harassment happened on r/nosleep, which is a reddit for sharing horror stories and creepypasta and so forth. In context, what you intended to be read seriously could reasonably be interpreted as an attempt to add to the story, and it looks like he responded in that spirit. That sort of exchange seems to be common on that Reddit, and no one intends them to be taken seriously. They're like forum games here.
As for the water foil board, that's an actual thing, although usually known as a hydrofoil board; it's a sort of surfboard with a hydrofoil array on the bottom so the surfer hydroplanes through the water and goes faster than they would on a normal board. Whatever the NSA presence in Hawaii might be, it's hardly surprising to see surfing equipment on an archipelago known for its waves -- but it would be surprising for the NSA to refer to waterboarding by a code that includes the words "water" and "board" rather than something totally unrelated and therefore totally unhelpful to someone listening in. That's been standard practice in the assignation of code words since WW2.
I grant you it's likely he's in the military, but there are some 36,000 military personnel in Hawaii. In the post, he mentions being assigned to a variety of administrative duties in lieu of the physical duties he'd be expected to do without his hip injury, which is exactly what one might expect of someone in the military. He's complaining about how much paperwork he has to do. Who here hasn't had more bookkeeping in our jobs than we might prefer? As for the budget review thing, review is not management. He's presumably expected to tally up the budget and ensure there are no arithmetic errors, not to determine where the money goes. Besides, we don't know how big a unit he's referring to. That budget may make perfect sense.
So, in short, what you regard as a death threat was made in a context where he probably assumed you were joking and was joking likewise, and nothing in his post history suggests he's anything other than he appears to be: a person in the military with unusual duties for his rank in view of his injury.