Okay. I found it. THe vehicle I was thinking about was called the "Alligator" or the "water buffalo". Officially it was an LTV-4.
The AAVP7A1 is the newest Assualt Amphibian in a series that started with the Roebling ALLIGATOR. The Alligator was developed over a period of 7 years, starting in 1932. The first "Gators" were a disappointment, in that the water speed was only 7.5mph. The land speed was 25 mph. Through design changes, and by using larger engines, the water speed of the Alligator was increased to 8.6 mph by 1939. In 1940, Roebling built a new model which was designated the CROCODILE. The Crocodile had a land speed of 25 mph and a water speed of 9.4 mph. The LVT-1 was a direct copy of the Crocodile, except that it was fabricated from sheet steel instead of aluminum. The LVT-1 was in production from 1941 to 1943. Being heavier, the land speed of the LVT-1 was 18 mph and the water speed was 7 mph. A 6 cylinder, 146 hp Hercules engine was used for power. The LVT-1 was propelled by tow endless chains fitted with cleats, both in the water and on land. The first LVT-1 were used as logistic support vehicles only. They were not armored and carried no armament, however, this soon changed. At the Battle of Turawa, bolted on armor plate was used and the vehicles were equipped with one to four 30 cal. machine guns.
The second generation of LVT'S was the LVT-2. This vehicle was developed in 1941 and was in production from 1942 to 1945. The LVT-2 was the basic design for a series of vehicles used during WWII. This family of vehicles included the LVTA1, LVTA2, LVT4, LVTA4, and LVTA5.
These vehicles were powered with 7-cylinder radial aircraft engines built by Continental Motors. These engines developed 220hp, their service life was very short. Major overhaul was scheduled for 100 hours, however few ever lasted that long. The transmission was a 5 speed, manual shift SPICER that incorporated a manually operated steer differential. This transmission had been developed for the M-3 light tank. As a result, the transmission was too-narrow for the LVT. This problem was overcome by using four final drives. The internal finals were bolted to the transmission/differential gear case and supported by two mounting yokes. The external final drives were bolted to the hull and powered the drive sprockets. This generation of LVTs was used through the Okinawa campaign in 1945.
This site has 2 photos of Alligators in operation. Roll your mouse over the photo and it changes.
http://library.thinkquest.org/TQ0312174/gator.htm
Here is a link that has several photos of a restored Alligator with the weapons shown. Great detail.
That is an Alligator.
Steve