From Supreme Commander Wiki
Tech 2 Amphibious Tanks are actually quite variable in both price and the role they play on the battlefield, but all of them are useful units in the correct situation. For instance, the Riptide is by far more powerful than other UEF land units, also costing more - its cost and DPS more in line with an Obsidian or an Ilshavoh, as opposed to the Pillar - the UEF's Heavy Tank.
However, being an amphibious tank, it's not quite as strong, DPS-wise as its land-based counterparts. With that in mind, in a straight 1-on-1 fight with either an Obsidian or an Ilshavoh, it will lose, as it has a shorter range and less health, however it does have more speed, often making it more useful in the backwards and forwards fights that occur in the early game. Similarly, other amphibious tanks tend not to give you quite as much DPS/Hp per unit mass as a land-based equivalent, and so careful consideration over which is best for the job at hand is always required.
Looking at the other amphibious tanks, the Cybran Wagner goes under the water, as opposed to above it - helping keep it concealed while the opponent does not have sonar, but simply leaving it exposed to submarines after the early game, its secondary torpedoes being all but useless in a fight with a submarine.
The Aeon and Seraphim Amphibious tanks fulfil much the same role as one another - they are cheaper than their main T2 direct fire units and more mobile - allowing them to fulfil the role of skirmishing tanks early on, however as the Aeon Amphibious Tank has a longer range and higher rate of fire than the Seraphim one, the Seraphim's hover tank is arguably the weakest of the four.
