An easy to follow guide on finding the Waterway Maintenance Kit locked space on Ashika Island.
Waterway Maintenance Kit Location Guide
This is another underwater cache. The fastest and safest way to this location is to zipline down the well just North of Tsuki Castle. Once you have zipped down drop straight into the water and you'll see its white outline. Of course, if you prefer to head through the Waterway you can, but you may have to contend with lots of armored Agents and a Juggernaut or two.