With stairs it would look something like this. The stairs would alas take up much of the ground floor and block the window view. Having enclosed stairs would be even more of an annoyance.
Discussion
I like the ladder and external elevator option.
The idea is that the below-deck apartment will have access to water-level deployment of boats. The whole Bond thing. 😄 If the mid section is compromised, an enclosed ladder or elevator would take you safely through down to water level where you could have means of escape. If electricity is out, ladder it is.
