They have only four possible basic speeds.
Near the end of the game, a nation can expect to discover the technology advance Nuclear Power, which automatically incorporates the "unit advance" Nuclear Power in future designs, increasing the speed by 1(00), i.e. to 5.5 when Steam Power has been discovered.
The only other speed increase available to all nations comes with Steam Power, likely to be discovered long before Nuclear Power is: it too gives an additional automatic 1(00).
A ship with basic speed of 4.5 will manage only 3.5 if its health is over 50% but under 75% and (as with all naval units) will manage 2.5 at 1% health.
The nation (if any) that owns Magellan's Expedition has all its ships travelling 2(00) faster than their rated speeds.