Im sure its totally different than the navy. Huge shock though?? Good or bad??? It cant be all bad if you've been doing it for 25 years.
Things will be done entirely differently than you are used to. The equipment will be nothing like you are used to, the work you do will bear little semblance to what you do now.
The consequences of an error are much different. I've yet to see a commercial power plant do a fast scram recovery after some tech left a switch in the wrong position, or turned the wrong valve. (Luckily I've never been the one to take a plant down, but tomorrow's another day and there is always the possibility.)
You will probably spend at least a year and maybe two in training. Pay attention and you'll learn a great deal, but it won't be enough. No amount of classroom training will substitute for actual hands on experience. When you do get the chance to do some maintenance, no matter how trivial you think it is, take it. You'll be watched closely by the experienced techs, DON"T take it personally. Their job is to make sure you learn to do the job the right way, without injuring someone or making it quiet in the turbine building.
All bad? Certainly not. This job is at times interesting, at other times mind numbingly boring. It isn't really all that difficult if you have any aptitude at all for it. If you don't you'll never be happy. If you are planning on going from the Navy to a maintenance position rather than an operations position, it is likely you have some level of aptitude.
Above all, DO NOT hesitate to ask questions. Don't think that you know how something is supposed to go, because "that's the way we did it on the boat".
No one is going to make fun of you if you ask a silly question. OK, they (we) probably will, but that's still a lot better than someone saying “hey did ya hear what Joe Blow did yesterday?"