I spearfish. This is a really non-buddy diving activity. We spearfish off the coast of Georgia. We are diving anywhere from 27 to 45 miles off shore at depths of 80' to 120'. Sometimes the currents are moderately strong. We do not anchor the boat and we do not put down boey's. Further, we are diving HP120s and sometimes make 1 to 3 dives off a single tank, so that the bottom time for a given time is not anywhere near fixed. A diver descending and finding no fish may relatively quickly ascend. The task of diver retreval rests entirely in the hands of those divers still on board.
Although I'm not aware of any unretreived divers, there was an occasion where a diver was lost and the coast guard had to come find him. For 2 1/2 hours the guy floated around and was passed over by the helicopter 3 times before he was located. I was not on that trip....thank God. It is not hard to imagine a scenario where a diver is just not spotted, the boat motor fails while divers are out, or extreme weather conditions arise while divers are down causing reduced visibility due to heavy rain, high seas due to wind, and possibly lots of noise due to wind or rain such that a whistle became ineffective.
I have another friend, who while diving in the keys, came up to find that his boat was draging anchor. Unfotunately, the buddy he left on board to operate the boat didn't know what to do when he couldn't get the motor to start. Worse yet, instead of letting out more rope, the buddy kept pulling and rethrowing the anchor. 1 hour and a mile apart later, the buddy popped a flare and a passing cattle boat came to the rescue of them all.
The point is, for about $1K, I can get a PLB in a diveproof canister. If I ever have to use it once, it has paid for itself. Also, I live and work in rural central Georgia. Cell phone reception is spotty at best. Keeping the PLB in my vehicle enables me to summons help to a car crash in seconds when cell phones would be inoperative. Again, a tool capable of saving someones life when time counts. Finally, since a PLB can call for Search And Rescue (SAR) anywhere in the world, it is a handy item to have on your person in case of emergencies while on land. (Camping, hunting, canoeing, river rafting, etc.) Once again, the PLB would give me away of quickly summoning emergency rescue and medical aid in the event of injury, critical illness, or once lost, where and when most cell phone simply have no signal.
As for PLB vs. EPIRB. PLB's are much smaller and far more adaptable to scuba diving.
The McMurdo Fastfind Plus 406 PLB with built in GPS has an optional diving canister that enables you to carry it to depths of over 100 meters. (Far deeper than I'll ever be venturing.)
I carry a small mesh poach on my BCD on my left flank behind my left arm. It is completely out of the way. It is my safety equipment poach. I have a sausage, whistle, mirror, pocket mask, night stick, and flare inside. I would love to include a PLB. Notice that the things I carry are only items which would be used on the surface such that quick underwater access is not necessary.
Here is a link to a very informative site regarding PLBs. I would suggest you each look it over before quickly dismissing PLBs off hand.
http://www.equipped.com/faq_plb/default.asp?Action=Cat&ID=1
Here is a link to a vendor's description of the PLB I am considering. Note the canister listed below the item.
http://waterways.tv/products/proddetail.cfm?prodID=4496&catID=na&subcatID=na&sscatID=na&mfrID=na
There is another product made in the UK which is much lower cost. The Sea Marshall. However, it is at the old frequency and does not summons aid by satelite. It's only good if the boat calls for the rescue team for you. Likewise, it would be of no use on-shore.
http://www.seamarshall.com/plb8ld.html