I would like to suggest the Panzerspähwagen Schildkröte III armed with a 15 or 20mm MG 151.
Currently german doesn’t have any amphibious Ground vehicles, while they can have a slight tactical advantage on some maps as well as can generally be a fun vehicle. Generally the selection of possible Amphibious vehicles (with guns) are very slim.
Aside that, it is pretty much completly inclosed as well as has the possiblity to mount the 15mm Mg 151/15 which with H-Pzgr. Belt, would be usable against everything, Infantry, Planes and Tanks, with great ballistics (1050m/s) and penetration (53mm/10m). And even if just with the Ap-I ammo (59g and 960m/s and 35mm/10m) it would still be better than the 2 cm KwK 30, because of fire rate, magazin size, velocity and aiming angles. (Also duo to how bad the 2 cm ammo is implemented.)
This Panzerspähwagen Schildkröte III would also have 2 more advantages which most other amphibious vehicles already in game don’t have. Great Land speed (its 4 wheeled) and enough armor to be immune from the front to normal MG fire.
History:
From 1941 the Trippel-Werke worked to produce amphibious armored cars which were based on already existing amphibious passenger cars. 2 Prototypes were build on the SG6 in 1942 with a 3rd ordered in late 1942.
These were called E3 (Einheits Serie 3) which were build from 1943 to 1944. These had different armaments and capabilities. The Serie 1 was armed with an MG 81Z, the 2 with either MG 81Z or MG 151 while the 3 had only the ability for an 15 or 20 mm MG 151 all in an rotating turret with AA capability. (One article also mentions a coax MG 34, on the 2) With the 3 Prototypes a turretless main production E3M was build as ammo carrier. However in October 1944 the Waffenamt decided that category of vehicle wasn’t needed and it was cancelled.
The III was powered by an Tatra V-8 Air-cooled engine with 80 Ps. With Armor from 5,5 to 14,5mm.
Overall the 15mm version would be better due to much greater penetration and velocity as well as its simply enough to fight the low BR planes. The II with MG 81Z would be a shredder of Infantry and Planes alike and the 20mm Mg 151/20 would shredd Infantry, Aircraft and get the Rank I light tanks, such as T-26. The T-60 and T-70 would still be able to be penetrated through the side.
Pictures:(Click to show)
I dont know which is II and III, the above and below one are different, the above has a round 1 piece wall under the turret/hull extention while the below one has a 3 Flat piece welded construction.
The Gun:
15mm MG 151/15
-10° to + 60° 250 or 500 rounds Loaded, 500 Rounds reserve either way. (Depending which standard ammo box like on the Sd.Kfz.251/21 is used)
The Vehicle:
Panzerspähwagen Schildkröte III (Basis the Trippel SG 6)
Speed: 95 km/h land, 14,5 km/H water
Engine: Tatra V-8 70 Ps
Weight: 5 Ton
Armor: 5,5 to 14,5mm
Crew: 2 or 3
Dimensions: 5,18 m / 1,9 m
Source: SG 6 Trippel Manual
Arms and Armour Press Encyclopedia of German Tanks
Handbuch der Flugzeugbordwaffenmunition 1936-1945