I think it's pretty cool. It's his body, and the artwork is done well. I've always enjoyed Mythology (Greek, Roman, Norse, and Egyptian), so I think it's pretty cool.
Poseidon was associated with the horse, a strong and powerful animal known for working hard, as well as the dolphin, a fast and intelligent animal. Two of the more noble creatures.
Even if this isn't why he got it, but he just enjoys Poseidon or how it looks, then that's his prerogative. For someone to consider it "utterly ridiculous" or act as though this is a mistake just reminds me of the old man yelling at the clouds from The Simpsons.
Funnily enough we were discussing this over dinner last night and my friend's 65 year old mother was talking about how ridiculous it is for people to get tattoos with no other reasoning than "tattoos are dumb and people just shouldn't get them". lol