San Carlos is the beach town associated with Guaymas. Guaymas is a prosperous town by Mexican standards and has produced 3 presidents of that country. The two towns are about 15 miles apart, close enough to go back and forth but far enough that if you start forgetting things it gets annoying fast.
San Carlos went through major development these last 15 years and there are condos, housing developments, multi-million dollar mansions, and major hotels on the beach. There are at least 3 decent RV parks with El Mirador having the best facilities but is also the most expensive.
The beaches are nice but they get a lot more use that those as Kino due to Guaymas being close and all the massive foreign build-up. This is good and bad, it is not quiet, but then they have an 18 hole golf course, shops, restaurants, and a night life both in San Carlos and in nearby Guaymas. There are hardware stores and car dealerships. This is the kind of infrstructure that is helpful when something breaks.
Guaymas itself is a bustling place. The statue of the fisherman is a must see as well as the plaza with the statues of the three presidents. There is a market just to the North of the plaza. Fresh tortillas can be had easily. There are two major supermarkets in town a Soriana and a Ley. Go to the Soriana, it has better prices, is huge with a wide selection, and is more locally oriented.
We drove South to Empalme, the next town down the road, just to take a look. Pretty quickly the standard of living falls like a brick and you are getting into poor rural Mexico. The contrast with Guaymas is fairly spectacular and not in a good way. On Sunday we went to the local flea market in Empalme and had lunch. Mostly junk for sale. I'd stick with Guaymas for the time and effort.