Sadly people can't handle the truth about alien intervention on planet Earth. Basically because they are scared.
I'm not talking about a spacecraft that's designed to go underwater. I'm talking about a craft that's crashed into the sea. It's wreckage lying on the ocean bed.
You are pre supposing that you understand alien technology. Very arrogant viewpoint. Don't talk about what you cannot comprehend.
If the spaceships can fly from other planets they are bound to be waterproof and one can only speculate as to the propulsion.
I'm guessing some form of nuclear reactor, possibly a small Nuclear Fission device, however, Space is a Vacuum and therefore not much is required by way of propulsion in space, water is a tad denser.
Solar sails that drift on solar winds. These sails will also work if the craft is on water. Gets a bit problematic under water though.
I don't know all about it. Every time they abduct me I understand a bit here and there. Normally they abduct me when I'm driving on the M8 motorway. Ever joined a motorway and the next thing you know your're leaving the motorway and can't remember driving along it like you were driving in autopilot? You've been abducted. FACT.