I asked this when I got a garage 8 years ago now. Loads of people said there was no point sealing the concrete floor, just use several coats of decent concrete floor paint.
Consequently, last year I spent a lot of time and effort sealing the floor with watered-down PVA (3 coats, I really didn’t want to have to do it a third time!), then repainting the floor.
Similarly for walls. Lots of people said just to use cheap emulsion. If, like me, you have single-skin brick or concrete walls which weep moisture even slightly, the emulsion will run or effloresce (i.e go powdery and brush off). So I re-did them with masonry paint. (Single-skin walls can get damp even with dampcourse and no obvious water intrusion).
For walls you can get external waterproof sealer. However, my garage abuts a neighbour’s garden and he has a load of stuff growing against the wall and wasn’t happy with me getting to the wall to do this.
http://singletrackworld.com/forum/topic/painting-garage-walls-and-floors