Might be worth a re-read of
THIS document (the water test is mandatory!)?
There's no such thing as a sealed box - small amounts of water WILL get in no matter (and you'll get condensation cycling). The simplest solution is to provide (protected) ventilation holes on the underside of the container to keep everything dry inside.
I usually bolt the LID of the box to the craft then use the box itself as the lid making easy all-round access to the contents and avoiding leaky box joints near the top. Cables should all enter from the underside (see above document) to avoid providing an easy path for water.
If your ECU and/or wiring has gotten wet in a salt water environment then you should thoroughly rinse it all with fresh water and leave it open to dry for several days otherwise salt will remain inside things and cause corrosion at a later date.