You want a DUAL BAND 406/121.5 EPIRB. The sat's will pickup the 406, and the planes/boats will use the 121.5 to homing i on you from short range. In addition a GPS enabled DSC VHF is an inexpensive, really good safety item for small craft. With DSC/GPS push one button and the radio will automaticlly transmit a distress signal with your LAT/Long to shore stations. If you're out of range of shore other vessels DSC's can recieve and relay your distress signal automaticlly. It takes about 5 secs to get a mayday out. Good stuff.
If you'regoing to get a maritime EPIRB, spend the $500 that a good one costs. Salt water is an increidably harsh environment and the PLB's aren't really design for it. Also a PLB's smaller battery could cause issues if it takes a couple hours to get a boat out to your location.