Is there a place where the sea floor descends into the mantle of the earth?
The subduction zones are more easily spotted by the phenomena they cause in the subsurface than their features on the surface. I found this schematic image which shows almost all the features, from https://www.researchgate.net/publication/318684905_The_Nahuel_Niyeu_basin_A_Cambrian_forearc_basin_in_the_eastern_North_Patagonian_Massif :


So what you see is the thin black plate subducting under the grey continental rocks. There are two features that it causes in an indirect way.


The first of those is the forearc basin, which is a sometimes present region with thrust faults in the direction of the subduction point. Thus, there is a local basin with a lot of space for sedimentation.
The volcanism, then, is the most obvious and always-present feature. All the chemicals that the ocean floor brings into the Earth at relatively low temperature create a 'melt' that can eventually reach the Earth's surface.


The reason why we don't see the actual subduction happening with our own eyes is because of the formation of the 'accretionary prism'. This is the triangle-shaped feature on the figure. It consists of sediment that is held on the interface between the moving oceanic plate and the stationary continental plate. It's a bit like a bulldozer. Hence, the real subduction action is covered by kilometers of piled up sediment! The deepest natural trench in the world, the Mariana Trench, is only about 11 km deep, while the continental plate is probably at least ~40 km deep. Hence, we don't really see the actual subduction in action...

If you have a shripwreck hulk and place it in the Mariana trench, it will get covered with sediment with a speed of ~0.02cm/year, so assuming your ship is about 30 meters high it will take 150 000 years before it is covered and can become part of the accretionary prism. Then it has to be dragged into the Earth somehow. The Nazca plate moves at ~5 cm/year, so it'll be moved by 7.5 kilometers forward. As I wrote, it'll have to subduct at least 30 more kilometers before it's below any continental plate, which would take at least 600 000 years at the speed of the plate motion, and assuming that it somehow moves at the same speed as the crust rather than getting bulldozered into the accretionary prism. In other words, it would probably take much much longer in practice before it reached far enough down. As said, that means that in the meantime your space ship will have been chemically weathered and mechanically deformed (by the enormous mass of sediments), and won't really be recognizable as a space ship anymore!