In a random series of thoughts this afternoon, it occured to me that both Gaius and Lucius Caesar died while at sea, shortly before their brother Agrippa became obsessed with fishing and calling himself Neptune. It also occured to me that the Emperor never got on a boat again, until the fateful nighttime voyage down the coast of Italy, during which he caught the cold that ended his life. Of course I'm not really suggesting there was an actual demigod pressing a supernatural grudge (not that there couldn't have been) but I do think the possibility could have crossed Augustus' mind in the more superstitious moments of his old age.
What I'm really wondering is whether this touches on Posthumous Agrippa's mental state. Suetonius says he had grown insane, which means Dio may be guessing to blame the new pseudonym exclusively on the new hobby. Either way, there's got to be something psychological going on when the last son of a dead admiral whose two brothers died mid voyage goes down to the water every day and proclaims himself master of the sea. I'm pretty sure Posthumous was the only "Neptune" Augustus really worried about, but it's interesting to see how the nautical theme must have helped hilight for the Emperor the personal instability which actually led to the young man's banishment in 7 AD.