Some benefit, but lots of unexpected drawbacks.
Everything "Amphibious" will immediately sink to the bottom of the water - because its behaving underwater as it does in air, and when I let go of something in air, it falls to the ground, which in this case is the seafloor.
A "non-amphibious" human swimming in the ocean is using an amphibious sword to slay someone - if in the critical part of the battle he drops the sword, it will fall to the sea floor (average depth on Earth of about 4km) at hundreds of km per hour, either shattering into pieces or embedding meters in sand depending on exact topology.
I hope the adventures in their quest to retrieve the magical amphibious sword of destiny from the seafloor have spells capable of simulating the correct gas densities and mixture ratios required to survive at each increment of their journey. (Sufficiently advanced magic should be indistinguishable from technology, after all). A similar problem comes up if you have an "Amphibious car" - either it immediately fills with water, or more air needs to be magically generated (and the gas ratios changed) as it drives down the hill of the continental slope.
Also note that an "amphibious boat" will not float - it will behave as if the water is air. An "Amphibious bucket" may not be able to hold water (as if it's not interacting with the water normally how does it collect it?). I may be able to use amphibious matches on amphibious timber to start a fire, but the fire is immediately doused by the water, or otherwise smoothers when oxygen can't reach it.
Some things require interactions with moisture to function: Ie "Amphibious superglue" will never set (as it needs to get some moisture in - usually humidity is enough). An "Amphibious desalination machine" or even "Amphibious reverse osmosis water filter" will function as if they're empty. A "Amphibious canteen" may never be able to filled up.