I've been to the Marriott in Key Largo but not stayed there, but have stayed at the Holiday Inn. The Marriott is a little nicer, but more expensive, than the Holiday Inn, but this is typical no matter what city you are in.
As the others said, neither is on the beach, but the Marriott does have a beach area on the sound/bay and is a good spot for sunsets. I didn't realize that Dive In didn't go to the "best" sites anymore, but it is a longer ride. To get to the ocean from the Marriott, the boat has to go south slightly and then "idle" down a passageway under the highway and into another sound. At the end of that sound they have to "idle" though another narrow passage. All in all, it takes I'd guess 20 minutes or so just
to get out to "open water" from the Marriott. So not that quick. It is much quicker from the Holiday Inn.
The wife did use the Spa services at the Marriott while there. She said the
massage was GREAT, but was unimpressed with the pedicure service. I'm no
pedicure expert by any means either, but I wasn't impressed at how "cheaply"
they had done her toenail "paint job" either.
The Holiday Inn is in more "central" part of Key Largo and has a lot of stores within walking distance (grocery, 2 drug stores, restaurants, dive shops, fast food, gas staions). It is however a Holiday Inn, so it's not as fancy as the Marriott, but also
doesn't cost as much. They do have a nice pool area with a TIKI bar between the
two pools. The onsite resturant was good also. It was also very nice being able to
walk next door to the dive boat instead of having to load up a car.
So I guess it depends on what you want to spend and what you expect out of a resort.
If you don't like the Marriott Dive In dive operator, Queissence diving services is
located next door to the Marriott and they have 3 boats onsite. They do a
GREAT job and they would go to any site we asked them to.
-Mike