That 21' Penn Yan looks like it would meet your needs - and it's already set-up for diving. The Johnson on it is a 2 stroke, so you're fuel consumption will be a bit high while your running. I'm not certain about that particular hull(is it a Spectra?), but there's a lot of dead rise in it. That's great for taking seas and having free board, but your top speed may not be what you want. The good news about the Johnson 2-stroke is they run forever. If that one has a PTO, I'd carry a spare, but the best thing for those engines is to run them.
Four divers and all their gear may be optimistic. I have a 25' Outrage and with a 225 I have trouble getting on step if I'm loaded with 3 divers and all the gear. I can do it, but it's my limit. With a full load of fuel, which is 1,000 pounds, roughly(140 gal.) I'm better off limiting my load. with less than 1/2 tank, I can add some weight. My boat easily packs 6 people, but with 4 divers(+120 pnds of lead), 2 kids, 1 non-diving adult and all the tanks we could fit(I think we had 11 steel), it didn't want to plane. Keep in mind there are a ton of variables, like prop pitch, bottom contour etc... But 4 divers who plan on two dives each is pushing the weight limit of a 21' boat.
At any rate, it would be worth contacting the owner to see how he dove it and what his weight limits were. Looks like the owner was all set up to dive, so he/she will probably be able to answer all your questions far better than we can by looking at photos.
Goood Luck, and let us know what you end up with. If you go look at the 21', tell us how it looks. If it was closer to Alaska, I'd be all over it.
Here's my boat after unloading camping gear on the beach last summer:
Also, that 21' Penn Yan with a single axle will likely fall into your towing limit. I tried towing my boat with a V-6 Ranger.... Laughable.... So I had to buy an F-350, used, of course...