Some tires are cheap, some tires are not, also all relative to budget. Maybe he plans to splurge on Schwalbe Marathon Supremes?
Anyways, everybody gave good advice above, pump it up more before submerging, slow leaks typically only emit one bubble at a time, and that at high enough pressures to cause the rubber to expand so the hole is "open".
Somebody else mentioned the valve though, I've had situations before where the rubber was cut at the base of the valve (sharp hole in the rim), and out of the tire the tube wouldn't leak air unless you bent the valve sideways to pull the hole open.
Something to check anyways...