Although the EPIRB may activate while submerged with the boat, it takes less than 4" of water to block the signal from reception by the satellite system.
This boater made a big mistake. When at sea, the ditch bag with EPIRB should come out and sit next to the captain within arms reach. Doubly so if you are going solo. I practice that religiously, mostly because of the risk of fire which can necessitate an instant abandon boat situation.
Corollary rule is everyone wears PFDs (inflatables) when on open water, and there is a PLB on my PFD's waist belt in addition to the EPIRB carried on the boat. (That same PLB goes in a canister with me for ocean diving.)
The sea is not to be trifled with, especially 80 miles out.