Hi
About cellestial navigation, yes, it'll be implemented, but the sky simulation etc... depends on finishing the implementation of the weather system, so it'll take a bit of time, but yes, it will be done
Another thing, regarding the currents, tides, etc... this isn't just surface currents, i mean, the way things are being made, there will be layers (altough limited to 2 or 3 layers though, since it's expensive to compute this), so you'll have thermoclines, and a bathythermograph. There's some controversy about use of bathythermograph aboard u-boats, but it seems they were indeed used (as you can see from the U-505 capture report, the US Navy intelligence officers did an inventary of all the equipment on board, questioned the crew, etc...
http://www.uboatarchive.net/U-505.htm
If anyone has any more information regarding this, it would be apretiated
The thermal layers probably won't make it into this release (next couple of monthes), but they'll be implemented (along with the weather system).
I'll post some news regarding the upcoming 0.4 release soon, stay tuned