Heat accelerates chemical reactions so colder water would react slower than warm. I believe deep water has less dissolved O2 as well.
Other than a cop stating it was an SMG, we have no idea what it was. A semi UZI, even if it has 3 selector positions, would look identical to a FA one unless you could function test it. Most of the cops I know have about as much knowledge of guns (other than the Glock they are issued) as Dianne Feinstein!
Could have been a widow or ex-wife getting rid of her hubby's collection "unofficially". Being MA, dumping them in a lake is probably the safest, least paperwork involved option.