It comes down to how well the EMP couples to the electronics. If you are on the grid and your TV is plugged in, the coupling mechanism is fairly solid and the TV is toast. Underway at sea or on a river the coupling is through an antenna of some type. The pulse may have enough juice to fry the radio or whatever device it's connected to, but the entire grid does not continue to feed it. There's a limited amount of amperage available through those topside antennas; any isolation should work to limit damage.

I really like the idea of back-ups to critical electronics. Handheld radios, laptop computers with the correct software. A back-up computer might be cheap insurance but many don't want to invest in insurance for something that may never happen -- sorta like flood insurance -- then a big hurricane comes and the dike collapses .... There's way too much unknown about EMP.

IMO, $.02