Marines A1-2D2M1C4,
3 units can be carried in an AP, but only 1 Marines Inf with 1 Artillery unit,
can be supported by Artillery unit 1:1 receiving +1A.
Marines Infantry get A2 after the first round of an amphibious assault (and all other rounds).
No combined bonus can make Marines A3, maximum is A2.
So, this way, Marines Inf will be better being transported for an amphibious assault than an Artillery unit.
But it cannot provides any combined arms support bonus.
This point make Artillery unit a better unit in regular ground combat.
And in a regular combat, Marines (4 IPCs) is not better than Infantry (3 IPCs).
Maybe any Marines Tank A3D3M2C7,
could act as Artillery for Inf or Marines Inf when paired 1:1 during the first round of an amphibious assault, giving 1 Inf or 1 Marines Inf +1A bonus.
Also, it could be carried in AP as a regular Tank with 1 Inf or with 2 Marines Infs.
What do you think?
Here is a reference about Amtank (LVT) being used during beachhead landings:
http://www.marineamphibians.com/Our_Machines.php
The First Armored Amphibian Battalion, U.S Marine Corps’s unit, existed and served in World War II in the years 1943-1945.
The First Armored Amphibian Battalion rode and fought in LTA s, officially "Landing Vehicle Tracked (Armored) but widely called armored amphibians, amphibian tanks or Amtanks. The LVTA had a turret mounted on a LVT hull, and more armor and more firepower than the amtracs (LVTs); the first LVTAs were developed for 1941 experimental swamp buggies with a 37mm gun mounted on them. The First Armored saw action successively, with two models of amtanks, LVTA-1’s for Kwajalein and Guam, LVTA-4 for Okinawa. LVTA-1 carried a 37mm gun and four .30-caliber machine guns. The LVTA-4 was equipped with a 75mm Howitzer, one .50-caliber machine gun and one .30-caliber machine gun.
The primary mission of the LVTA’s was beach assault. They were to lead the first infantry wave across the reefs, firing enemy targets until the infantry hit the beach. Since in that phase of the landing, naval and air bombardment had been lifted, the LVTA’s provided the last firepower against hostile beaches before the troops were actually ashore. Secondary mission, after the initial beach assault, included beach defense and amphibious patrols. With that at advent of the LVTA-4, with its 75mm Howitzer, it became possible for the First Armored to function as artillery, a new mission conceived of and developed by Major Louis Metzger, the Battalion Commander. As artillery, each tank platoon was organized as a 75mm Howitzer battery for indirect fire on enemy targets. The First Armored could thus field three artillery batteries for each line company, a total of twelve from the whole battalion.
This link provides many pictures and other links on Marines Tank:
http://www.ww2f.com/topic/19554-inquiry-us-marine-corps-armour-in-the-pacific-war/
This is a very interesting point on how various Marines arms were working together with Navy Battleships:
The Marines made great strides towards refining supporting arms coordination during the battle for Okinawa. Commanders established Target Information Centers (TICs) at every level from Tenth Army down to battalion. The TICs functioned to provide a centralized target information and weapons assignment system responsive to both assigned targets and targets of opportunity. Finally, all three component liaison officers artillery, air, and naval gunfire were aligned with target intelligence information officers. As described by Colonel Henderson, the TIC at IIIAC consisted of the corps artillery S-2 section “expanded to meet the needs of artillery, NGF, and CAS on a 24-hour basis . . . The Corps Arty Fire Direction Center and the Corps Fire Support Operations Center were one and the same facility with NGF and air added.”
Such a commitment to innovation led to greatly improved support to the foot-slogging infantry. As one rifle battalion commander remarked, "It was not uncommon for a battleship, tanks, artillery, and aircraft to be supporting the efforts of a platoon of infantry during the reduction of the Shuri position."
http://www.nps.gov/history/history/online_books/npswapa/extContent/usmc/pcn-190-003135-00/sec5.htm