Updated 2023-05-03 !
In a former post I made a writing about Shelly 1(S1) and this post will handle about the successor Shelly PLUS 1(SP1) but much of the content is also valid for some of Shellys other devices which uses the same software/API.
To keep it simple, SP1 is a WiFi switch, 16 Amps, with dry contacts but compared to the S1 the hardware and the software is updated.
The function "without Internet" + that the SP1 can use a 12V DC source makes it perfect for an "use case" in a boat/mobile home.