This is perhaps a somewhat unpopular opinion, but I'm always pleased when I hear ultra-rich people spending exorbitant amounts of money on something.

Without going into the discussion of whether or not people should be able to accumulate huge fortunes in the first place, given the fact that they exist, such spending means that they are putting huge sums of money back into the hands of "the people": In exchange for this boat, Bezos is moving 500M$ from his bank account to the bank accounts of many, many people working on it directly and indirectly thus moving the pendulum a tiny bit towards financial equality. Considering that the real-world alternative was for him to not have done this, remain under the radar, and keep 500M laying around, I think this is the better option.