My N900 got submerged, and although I dried it up completely before trying to turn it on it won't boot further than the Nokia screen, and after a while shut down and then boot to the Nokia screen again.

When I connect it to my computer and simultaneously press U the USB logo will show up on the Nokia screen, and the computer (Windows Vista) will say that it has detected the device, but I'm not able to see it in My Computer, so atleast it's not mounted as an external drive.

Is there any way to rescue the data on it?

I would be very glad to hear any advice you might have!

You don't mention if you took out the battery immediatly after it got wet. If you didn't it might be toast.
Failing that if all else fails, flashing with the Vanilla image might help - NOT the EMMC as that will wipe your data.

I did take it out immediatly, so let's hope it can be saved.

Thank you, I will try to flash it with the Vanilla image.