When I retire I just want to buy a nice houseboat designed for me to use as my home.
Have you ever spent an extended amount of time on one?
Yes my uncle best friend actually lives out of one now his is under operation of their own power which I would need as well.
It seems like a lovely life, but I'm not sure I'd be able to do it full time.  I'd be worried during storms.
That is one of my main concerns because you never know the outcome or aftermath of the storm but the houseboat has different sheds etc used for storing of the boats.
Like a marina in a lake?
Somewhat but not quite it depends on the location I would be on the beach in my bout so the marina at the beach is more wide and has rocky formations so different settings. 
Oh a houseboat on the ocean?  I"m sure you'd see some amazing sealife, like dolphins!