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I can't comment on the Marriot but I have stayed at the Holiday Inn, (my LDS books short trips a couple times a year and SeaDwellers picks up at the dock right behind the hotel).
The Holiday Inn is just ok, its dated, rooms and bathrooms are small, has a motel feel to it. There is a decent pool/hot tub/tiki bar area though so you don't have to hang around in your room. It is very close to a lot of good food and they do have a breakfast buffet that is one step up from the usual. If you are just looking for a place as a base of operations and not a luxury stay it will do fine.
Key West Inn underwent a renovation .. thus higher $$$.The "Key West Inn" is next door to the Marriott. It's much cheaper than the Marriott if that's what you're after. The beds (and everything else) is much nicer in the Marriott, but you pay for it. You could easily walk from the Key West Inn to Rainbow Reef (or drive if you so choose).
Down the road on the other side of road is a Hilton of some sort. I've stayed there too. I had problems with the A/C when I stayed at that Hilton (even after they moved me to a new room). The grounds are quite nice, but having got two rooms with basic maintenance problems I won't go back to that Hilton.
Edit: It appears the Key West Inn has been renamed to "Waterside Suites & Marina" Key West Inn | Budget Resort Hotel in Key Largo, FL
Also, prices appear to be far higher now. Maybe it's because it's high season, or maybe it's under new management and they've lost their minds. Last time I stayed there, I got a room with a king bed for $129/night. Currently they're listed at $250. The Marriott is only $229/night for a king right now so don't bother with the Key West Inn. Must have new owners that are on some kind of drugs.
I think a 5 mm in April is a bit overkill . The water temp should be
80 degrees on May 1st . Good luck and safe diving !
Sea Dwellers