You activate the "Submerge" option, by clicking: "Submerge" down at the left under the icon.
Activate submerge using target option to permanently reveal camo bloons in its radius.
~ Description from BTD 5 iOS
Submerge and Support is the third upgrade on path one for the Monkey Sub. It costs $425 on Easy, $500 on Medium, $540 on Hard and $600 on Impoppable. It adds a new targeting option: Submerge. When selected, it ceases firing darts, however, it will send out green concentric circles that will remove camo from bloons. Unfortunately, once submerged, it is submerged until you set its priorities to one of the default (First, Last, Close, Strong).