If you ever used a handheld vhf on a boat, you know its range is severely limited compared to mast mounted units. The rule of thumb is typically 1 watt per nautical mile, but realistically you can rarely get more than 2 miles at deck level, say three or four feet over the surface . Now try it in a mere foot or two of swell IN the water inches over the surface. I doubt you'll get more than quarter mile or so of range given that vhf radio waves are (for the most part) line of sight dependent. So basically the only boat you would be talking to is your dive boat. If you were drifting in a five knot current, you would be over 5 nautical miles away in one hour way beyond the range of any boat if you were using a typical handheld vhf while at the surface.
In addition, you would have to go on the assumption that 1. The dive boat is within that limited range. 2. The boat has the vhf radio on. 3. The vhf is monitoring channel 16. 4. The boat has the volume turned up. 5. The skipper can actually hear your mayday call over the sound of the diesel engines, the wind, and other noise.
The current nautilus lifeline receives its gps position via satellites and transmits via a protocol called DSC to all boats with vhf radios. While the USCG requires all new fixed radios to be equipped to receive DSC calls older radios may not be equipped with this feature. Most handhelds do not have DSC capability though more and more do. When an emergency call DSC signal is received, it triggers a relatively loud alarm on that radio and typically shows a graphical picture of the signaler's position on an lcd screen relative to the boat. However, you still are limited by line of sight range and the fact that the unit emits less than 2 watts of power in its transmission. And once again, you must once again assume the skipper has a fixed radio equipped with DSC capability, that the radio on, and that he can hear the DSC alarm... lots of variables still!
So much better to use a PLB that sends your gps location directly up to satellites where the signal is then very quickly relayed to ground stations and then to the appropriate rescue agency.
And yeah, lots of countries require a radio operator's license to transmit on a vhf radio with the operator having to take a written test to acquire one.