Just going to compile all information I can find on this for reference.
Voyage Ship Exp
x5 Rosa + 1 Herb Town = 2800 exp (7 bosses)
x4 Herb Town = 3600 exp (8 Bosses)
Drop rate increases don't affect chances of boss drops. The drop is done via invisible chest, which is just pure luck.
Pretty pathetic you only get 4 inventory slots until the final ship which gives another pitiful 2. At least the energy goes up by 20.
A boss is added for each map you progress through. Permitting high exp while not sacrificing SW attempts.
Rosa x1 Green Kraken
Herb x1 Green Kraken, x1 Green Kairyu
Rien x1 Green Kraken, x1 Green Kairyu, x1 Red Kraken
Lith x2 Red Kraken, x1 Green Kairyu, x1 Blue Kairyu
Big thank you to this guy for his information: http://uni-wz.com/blog-entry-2236.html