Amphibious vehicles have the advantage of not being hittable targets while underwater, unless the enemy goes into the water after them. Basically, a hover generally has the speed and maneuverability to avoid most of the incoming fire, but anything that does hit has a depressingly high chance of immobilizing the vehicle, which will then sink, along with its crew and passengers. Using the hovers as a distraction right before contact, until the amphibious vehicles can clear the water and disgorge their passengers, might work. Loading up one or two amphibious vehicles with torpedoes might be a nasty surprise if the defenders DO decide to go into the water. Picture an amphibious SRM Carrier with torpedoes, and the ensuing armor breaches. OUCH!!!
As pointed out, hovers have the worst motive crit table, followed by wheeled, with tracked vehicles suffering the least (but STILL highly vulnerable to an SRM rack or LBX cannon).