My $0.02 worth...
The explanation the selling agent provided was that the owner paid for an OceanFront unit but Marriott assigned him a Unit Number that was OceanView but that he is treated like an OceanFront holder.
Personally,
I wouldn't just blindly accept a selling agent assertion of "how the seller is (allegedly) treated" in terms of unit assigment, as it seems to me that the unit number / view
on record within Marriott would ultimately prevail and is is likely all that will really matter or apply (to you) in the future. The selling agent
could perhaps just be contriving a factually inaccurate "story" in order to not lose the sale entirely.
It sounds like the seller agent is asserting that the owner somehow got erroneously "assigned" something other than what they
thought they had purchased, but (...if that's even true in the first place) the owner perhaps never subsequently got the (alleged) error
corrected on the internal Marriott records.
I'd proceed slowly and carefully here --- and I would do
whatever it takes get a first-hand Marriott version / position on this discepancy on unit identification
before proceeding any further toward a closing...