I've built a couple of garages in my time, but never slept in them (smile)
How you design and build the garage depends on a number of things: the most important of which is where the lake is. A garage build on a lake in Florida will be much different than a garage build on an Alaskan lake. Other factors you need to address is size: Do you want enough head room for an RV, a pick-up or just a passanger car and how many (single, double or triple car garage)? Do you want a permanent or temporary heater? Will you be generating your own electricity or tie to the nearest town's power lines? Do you want windows? Skylites? Will you be mixing your own concrete or having it trucked in? How much work do you plan on doing yourselves? Where do you plan on getting your building materials: local or haul them in?
Providing this (and any other info you can think of) will help those of us who have done this to help you :-)