Longboats are good to get back water or to fast build a large navi because they are created faster than galleys.
I am not shure if it's worth to build turtle ships because they are so expensive. You may wanna read this: http://ageofempires.wikia.com/wiki/Turtle_Ship
I am not shure if it's worth to build turtle ships because they are so expensive. You may wanna read this: http://ageofempires.wikia.com/wiki/Turtle_Ship