What does the Submarine do in BTD5?
Solution 1:
http://bloons.wikia.com/wiki/Monkey_Sub
Placed in water, it throws darts on bloons at medium speed(1.32 times/sec). It can be upgraded to have nearly unlimited range (by being able to target all bloons within the radius of any other tower), detect camo bloons, and to launch missiles (with an ability able to destroy up to a ZOMG in a single blow).
This tower has one of the best power/price ratio, and is the only one able to one-shot a ZOMG.