Anna,
I don't know how long the actual words spoken (in Great Britain at any rate) have been in use at a launching ceremony, but the ceremony itself is of some antiquity. Found this on Wikipedia:
http://en.wikipedia.org/wiki/Ship_naming_and_launching
If the ship is launched 'end on', this is usually done stern first in the UK. However, in some European and other countries ships are launched bow first. I don't quite know why the difference, although personally I would have thought that a bow-launched ship was less managable. Ships began to be side-launched in the nineteenth century, perhaps the most famous example (the first?) being Brunel's 'Great Eastern' on the Thames, which took several attempts before it entered the river. There used to be a plaque marking the spot. A good diorama model of this can be seen at the NMM, where one could spend some time discovering the various other things going on besides looking at the ship herself! There are also several photographs taken at the time, perhaps the most well known being of Brunel himself standing, I think, in front of the enormous chains used for the launch.
Nelson would perhaps have been familiar with a ceremony of sorts, although not perhaps as we know it, but I am not aware that he attended any such event. The larger naval vessels were usually 'floated out' rather than launched, since they were built in the dockyards in a dry dock. Smaller vessels such as frigates, which were often built in private yards under contract, would have been launched in the traditional way.
The reason why you can't find the 'o' with an oblique line through it is that you very likely don't have a Norwegian keyboard. It's not on my Swedish one either, which has instead three keys marked å, ä, and ö, which are used in Swedish – or so I'm told!