About the best you can do is a boat that is fundamentally buoyant or (aka positive buoyancy) via a combination of materials and sealed spaces, which basically m

About the best you can do is a boat that is fundamentally buoyant or (aka positive buoyancy) via a combination of materials and sealed spaces, which basically means you can submerge it, flip it, fill it with water, and it buoyancy is enough that it will come to the surface. But if you overload it or damage it and compromise air spaces or break off or crush lighter-than-water materials (e.g., styrofoam filled fixtures and voids), then it's no longer unsinkable. So you're right, nothing is unsinkable. Not even when "operated properly" and maintained properly, there's no guarantee you won't run into unforeseen conditions. An unsinkable boat is as ridiculous as an uncrashable airplane or automobile.