They didn't use them for landings. The Marines had lots of other amphibious stuff that would actually float in the ocean.
JR
Certainly their primary utility was moving supplies and evacuation of wounded over difficult terrain, however there are some passages of interest from "Marines in World War II Historical Monograph" "Iwo Jima: Amphibious Epic" by Lt. Col. Whitman S. Bartley, USMC.
In chapter 2 ("Plans and Preparations") we have the following information:
"Among the great variety of amphibious equipment available for landings, two items were new to the VAC: the Clever-Brooks 3 ½-ton amphibian trailer, and the light cargo carrier M-29C (weasel), capable of hauling a half-ton load.
<SNIP passage describing amphib tractor>
The weasels were received and distributed to the three divisions in November. Outstanding performance in tests and training exercises disclosed that these boatlike tracked vehicles resembling miniature LVT's without ramps, were valuable equipment."
Also, in footnote 55 in the same chapter:
"55. The fast, maneuverable weasel, with its low silhouette, pulled trailers and artillery pieces over the terrain that wheeled vehicles could not negotiate. Of great value on land, they did not prove very seaworthy. VAC Logistics Rpt, 45."
So, though these passages are open to interpretation, I think it possible that the Weasels were floated ashore (as opposed to being carried in an LCM, say). Certainly they would not have come ashore until the beach was secured.