Packless Shaft Seal (PSS)
I have rebuilt my fare share of industral water pumps that uses the same type of seal as the PSS They have been using them for as far back as I started rebuilding pumps (1969) and probably longer. The thing about them that I don''t like is when they fail, they fail right now with no warning. And then the shaft has to be pulled to install a new one. The same on a boat. At least with a stuffing box (packing gland on pumps) one can put some more packing material in and tighten it down some more, then do a good repair or replacement when you get back to port.
I vote for the good ole stuffing box. Carbon seals are for high speed, like power boats. And they recommend water injection anyway. As for sailboats, if you have one a little bleeder valve would work to get the air out, or just pull the boot back just enough to burp it then release.