Boats. Per @Ash, an "amphibious" boat won't float. But if the amphibious boat has a small hole in the hull, and a normal boat floats inside the amphibious boat, and the amphibious boat is held up by the normal boat, then it will be held level with the water line, and it will move through the water without resistance, and the normal boat will sit in the midst of its little lake of co-moving water without needing to fight any resistance at all. So the only requirement is a double hull. Then the normal boat can have a normal screw protruding out the holey hull, which interacts with the water normally to provide propulsion unopposed by drag.

Nets. The "amphibious" net moves through the water with the greatest of ease. Dragged behind the boat above at a hundred knots or so it should be very effective (you may have to slow down a little to make sure you don't lose your net to a dolphin or press fish fillets through the holes).

Matches. Nobody expects a frogman (or frog-man) with a magic match that works exactly the same in water as it would in air. If you can wipe your nose with a magicked tissue underwater, you should be able to do this. The flame produced is made from the material in the match, so it contains the same "amphibious" component, so I suppose it could engulf the victim boat from underneath. Trippy.

Seaplanes. A gift fit for a Kennedy, these delightful aeroplanes don't need to stop at altitude zero. Yes, yes, @Ash reminds there's no air to power the engine, which is why you start it with a magic match.

Boats. Per @Ash, an "amphibious" boat won't float. But if the amphibious boat has a small hole in the hull, and a normal boat floats inside the amphibious boat, and the amphibious boat is held up by the normal boat, then it will be held level with the water line, and it will move through the water without resistance, and the normal boat will sit in the midst of its little lake of co-moving water without needing to fight any resistance at all. So the only requirement is a double hull. Then the normal boat can have a normal screw protruding out the holey hull, which interacts with the water normally to provide propulsion unopposed by drag.

Nets. The "amphibious" net moves through the water with the greatest of ease. Dragged behind the boat above at a hundred knots or so it should be very effective (you may have to slow down a little to make sure you don't lose your net to a dolphin or press fish fillets through the holes).

Matches. Nobody expects a frogman (or frog-man) with a magic match that works exactly the same in water as it would in air. If you can wipe your nose with a magicked tissue underwater, you should be able to do this. The flame produced is made from the material in the match, so it contains the same "amphibious" component, so I suppose it could engulf the victim boat from underneath. Trippy.

Seaplanes. A gift fit for a Kennedy, these delightful aeroplanes don't need to stop at altitude zero. Yes, yes, @Ash reminds there's no air to power the engine, which is why you start it with a magic match.

Watering cans. Have you ever dropped a match on the sea bed and accidentally caught the weeds and debris on fire? Obviously you need an "amphibious" fire extinguisher to put it out, but it might be simpler to have an amphibious watering can. Fill it up with water on the surface, and it should still work the same way below water, right? So pour your water out of the can and it will splash down and put out the fire. Keep one on your boat in case there are frog-men about. You might need one (filled with fresh water, of course!) to water the "amphibious" corn kernels that you're farming on the bottom of the ocean; I'm not sure.