Equip your ship with fire arrows and stuff and set enemy ships ablaze during a ship battle. you need a field of "adrenalin" to fire fire
important note:
it SEEMS to me that this isnt about smoking 10 ships in a row whatever ship it is. 10 spartan light ships didnt do the job.
my progress stalled while i filled the ocean with shipwrecks.
BUT!
every time i sunk a new kind of ship i got another 10%
for example:
spartan light
spartan heavy
greek light
greek heavy
merchant
ship with whatever sail it is in blue
pirates
you got it.....