Probably not as many as you'd think.
Most M1's went over in the hands of the troops who were usually issued the rifle in the States.
Unless a troop ship was hit, most made it over, and few troop ships were ever lost in the Atlantic.
I'm sure there were some shipments of packed rifles put on freighters, and we did sent some to the British later in the war.
Unlike the Thompson gun, we didn't ship huge loads of M1's around.
Unfortunately, MANY Garands went to the bottom of the Atlantic off Normandy, in the hands of the brave 29th IN Division's heroes & brave men of other units that died on D-Day, even prior to touching France. - Better men that I will ever be.
(I was once a 29er, in the VA ARNG and I have personally seen the casualty lists of that LONGEST DAY.)
Otoh, I agree that few supply ships and loads of Garands went to the bottom in the war in the Atlantic Ocean.