The Rambam (Hilchot Mikvaot 2:11) does not allow loosely holding a utensil in a mikve
When one holds another person or an implement and immerses them, they remain impure. This applies even if he loosens his hand and allows water to reach the person or the article being immersed. This is a decree, lest he not loosen his hand. If he rinses his hand in water first, the immersion is acceptable.
(see also the mishna in Mikvaot 8:4)
However the Shulchan Aruch (YD 120:2) rules that it is permissible to hold a utensil loosely
The vessel must be loose in your hands at the time of dunking, for if it is tight in your hands it is an interposition, and if you dampen your hand in the water at the outset there is no need to worry (Rema: and specifically that you dampened your hands in the waters of the Mikvah but not in water that is detached).
Why this difference of opinion? What led the SA to rule differently? Is there a difference in the physical reality (e.g., were mikvaot at the time of the Rambam so deep that they would be worried to loosen their grip on utensils?)