[A_Brandybuck]

I've ever thought of this dream with the sea of being foresighted. It is already very odd, that a Hobbit have a desire to see the sea. So, the dream must have a different meaning and then it fit very well the fact that Frodo will go over the sea to the Undying Lands.
The white tower could be the tower, where the unique Palantir is laired, which can only see not communicate.
Climbing the tower and look upon the sea could have meant to use the Palantir and have a look of what Frodo would expect after his mission, although it is still the beginning of his adventure.
I don't have my books here, so I don't know whether the thunder and the flash are part of the dream or already the connection to the real world. A sign that Frodo wakes up.
If it is part of the dream, I could imagine, that this could be a sign, that the future is unknown. It is not for sure, that he could make for Valinor. He must first finish his task.

That is how I would interpret the dream.