These tiny gearmotors are available in a range of gear ratios. All of the micro metal gearmotors have the same physical dimensions, so one version can be easily swapped for another if your design requirements change.
This is the waterproof inspection LT2 Robot. It has flipper arms to help it climb obstacles and for better maneuverability. It is capable of wireless or tethered control. The tethered option includes a fiber-optic cable spool up to 1km long.