Fiberglass reinforced body filler does
not absorb moisture. It is recommended for boat/marine use. It does not need to be sealed. It makes a strong repair.
From experience I can tell you that this repair does work. Also, from experience, I can tell you what to expect.
The reason I told you that it needs to be redone in a few years is because the corrosion that has already started will progress and cause the body filler to start coming off around the margins of the repair as the corrosion works it's way under the repair. Spraying the entire underside of the deck with epoxy primer will delay this process but it will also wear off.