I was actually curious about that. A form I was picturing for Poseidon in this day and age was something close to this. 

[hider=Potential Poseidon]
http://hairstyleonpoint.com/wp-content/uploads/2014/12/49cd4c44d562e92d90a3a4c81ced02dc.jpg
[/hider]

Didn't know if this was too far off from what you had in mind or not? Thought it would give off a more approachable vibe to whomever summoned him.