I think that the dream about the sea can be very straightly understood. I don't know where the dream came from, but clearly it woke the longing to the sea in Frodo, though deep in his heart.
This may have been some kind of call, maybe sent by the valar, Ulmo or the Sea itself.
And as in "real" world (I mean our world) you may dream about something you've never seen, only heard of. Then we may ask how could he imagine it look like it really looks like, maybe
this was sent by powers mentioned before.
I have one more idea, but it's a bit far-fetched. Maybe the sea came from Frodo's subconsciousness. (The strange idea: he had it in his subconsciousness because he had seen the sea before he was born. This brings us to different questions and the structure of Eä and where do the souls of hobbits go after death and where are they before they born, but my intention wasn't to mix this intresting thread up, so forget this theory. It doesn't even agree with Arda itself, I think.)
Sorry, I only mixed this thread up, but that's may way to do things
(And sorry, my poor english, that's not my mother tongue. Hope you understood something.)