could they make a Magnetohydrodynamic drive?

Question

how effective would it be?

Answer 1

Yes, an MHD drive can and has been made. Unfortunately, it is not very effective as it requires large amounts of electricity ( for a seawater MDH drive ). Although MHD is quiet, a generator for electricity is not and this defeats the main MHD attractiveness. Here's a quote from Wikipidia:

The major problem with MHD is that with current technologies it is more expensive and much slower than a propeller driven by an engine. The extra expense is from the large generator that must be driven by an engine. Such a large generator is not required when an engine directly drives a propeller.