Anyone here into boating?

Been thinking I'd like to pick up a decent second hand boat with a trailer someday that I can use in both fresh and saltwater. Ocean is about 3 hours a way, but do have some decent fresh water bodies around (including the CT river).

Would be looking for something that could comfortably hold 6 for a daytrip/fishing. but wouldn't need any sleeping accomodations; although a place to get out of the sun or poor weather might be nice.

Anyone know what the costs are to maintain a boat? Besides the initial purchase, is their required insurance/registration fees etc like a car? Can you operate it in multiple states(like a car) or do you need a permit from each state?

What size boat would be big enough, yet easy enough to trailer around load/unload and operate?

I am starting from ground-zero on this one so any advice is welcomed.