The Speed Boat is very cheap and can carry up to 5 troops and take 

One Army Unit

The builder boat is a way to build in the water.  Not as useful as the builder as it can only build the harbor and oil rig. However, the oil rig is very useful for the endgame so it is worth getting.

One Army Unit

This boat is able to carry up to 10 units, not just troops. It can have vehicles and troops. Although it is slower than the speed boat

One Army Unit

This is a water attacker, it can move throughout all of the water and attack anything in the water. Ranging from other subs, to oil rigs, to even harbors. This is a high damage water vehicle but the downside is it is only in the water.

Three Army Units

This is a land and water attack, however it can only stay in water. Not doing as much damage as a sub but having a faster attack. 

Three Army Units