How could the truck GPS and phone be off around the same time?
1) Tom intentionally turned both off.
2) Someone else turned them both off.
3) They are both underwater and inoperable.
Looking at the area on Google, I do not find much water access that would be undetectable- knocking down a fence or leaving tracks in the sand.
However, there is the boat ramp just next to the Katch. Attached is an overhead image of the Katch (Venetian Shores) with ramp in lower left, and a street level view of the ramp area. The thing with this boat ramp is that it is paved, just as a road is paved. Add drinking and maybe impatience and the unlikely becomes more likely.
I'm surprised a sonar boat search has not been used yet.