I know one guy in the Navy reserves in communications. I don't think he has ever gone on a ship. He seems to really enjoy it and so far hasn't been called up for active duty.
I know someone else in the Coast guard reserves in the Great Lakes. He has done far more then just the weekend a month and two weeks in the summer but I'm not sure if he is considered active duty. He is often on a boat but really seems to enjoy it.
But, yes get everything in writing and really think about it, you could end up doing far more then what you had in mind.