Lots of little creatures that looked a lot like him with odd coverings and really really tiny.
Personally, I'd like to think the Big Guy didn't pop up just to say "Hi".
Maybe he was lost.
It's tricky getting around underwater without a map or road signs. Only having the fish around him, reknown for not being the wittiest of conversationalists, upon peeking out of the water and seeing much smaller, tailess, versions of himself, I suspect Godzilla came to land to ask where he was and what was the best way to go wherever it was he was going.
Human nature, lovely thing that it is, the people of Japan didn't do the sensible thing and help this guy out. No! Instead, they started launching a bunch of missles and lazers and other less than pleasant things at Godzilla.
All of this conflict, years upon decades upon (you get the idea) could've been avoided with just a little common courtesy.