Even a "wireless" transducer
needs to get power somehow, so they're not really wireless.
B&G H3000 system has a provision for using two speed transducers. I'm not sure about the depth.
It would seem that a 21' catamaran would need only a few inches of water to float--do you really need a depthfinder? As others have suggested, a handheld chartplotter
for your area may be all you need.