Check the literature section for the brochures, which will have the specs for all the models. With the I/O, I wonder if it's the Seafari cuddy cabin model? If so, IMHO it's the best riding of all the 20' models because it's better balanced with more weight forward. There is also a good Boating Magazine article on the Seafari I/O with an excellent description of the construction details in the literature section. To fully appreciate a SeaCraft, check out the article on SeaCraft founder "The Amazing Mr. Moesly" in Literature section.
1972 was the last year for the original 4-stringer Moesly design, which I personally think is a little stiffer than later versions; Potter switched to 2 wide box-section stringers in 1973, I believe in an attempt to take some cost out of the boat. There have been no reported problems with either design however; the stringers are all glass in either version. The I-6 volvo would be my favorite I/O because of good reputation for durability and smoother than the I-4 Mercruiser.
The Mako will ride like a flat bottom boat by comparison! The SeaCraft is probably the softest riding 20' hull going, except maybe for the heavy extreme-deadrise 20' Bertram, but it'll burn much less gas than a Bertram. IMHO the Seafari 20 offers a combination that's hard to beat: light weight but Bahamas-capable seaworthiness, easy to trailer, fish/dive/cruise/camping versatility, all with 4-5 mpg economy with I/O or DI 2-stroke power! Haven't seen any modern boat that matches that. In 4-6' seas, you'll have to slow down if you don't want to go airborne, although going airborne is no big deal as it'll still land soft if you keep it level. It actually rides harder in the 2' square waves often found on the Little Bahama Bank, but that's no problem if you have trim tabs and are balanced so you can plane at 12-15 mph; in that situation, I've run thru 3' waves very comfortably, SITTING DOWN! The SeaCraft hull is built like an airplane and very strong, but it's relatively light, so it'll start to go airborne in 3' seas at about 20 kts.
If you're gonna spend a lot of time in 6-8' seas, the old
Moesly 21 would be perfect (can stay on plane in 10-12' seas; Moesly RACED them in conditions like that!), but they're so rare you'd have to be extremely lucky to find one. The 25 Seafari would be my next choice for a soft ride, but it's a much bigger boat! After that, the 23 will also ride better than the 20, just because of more weight and deadrise, but anything bigger than the 20 will be much heavier, require a tandem trailer, bigger tow vehicle, and 200+ HP and more gas.