Yup, river police are feckers but for the good of all mankind, once you get out of the bay / estuary / river mind you're free to do as you please.
Lifejackets are a MUST, you also have to familarise yourself with wherever you are taking the boat, I know around my way there are lots of cliff edges, underwater rocks etc
Understanding tide times is also a must, launching a boat at low tide or being out during low tide can get you in some serious trouble.
Stay away from the coastline unless you know it / know what you are doing.
As above, fisherman will have your head off and cast lines can be hard to spot untill it hits your boat or you see a nutter shouting and waving on the shore line, then you know you're about to take his line out and he won't be happy.
In and around beaches ( if you're aloud) can also be dangerous with animals and people in the water, they can often be hidden behind a small wave or ripple or even the break.
my number one tip, is when you get the hang of it get yourself a doughnut and a solid state bar so you and your mates can learn to water ski.
It's A) hilarious to watch newbies face plant and B) awesome fun when you get up and running
Also watch out for sewer pipes etc you don't want to be in the water near those you'll end up with some nasty skin conditions, I've seen all sorts of nasty looking things from friends that surf.
I would also recommend a quick towing course if you are new, it will make your life a million times easier if you can reverse the boat down the slip rather than detaching it and walking it down but some can be very narrow and I would hate for you to lose your boat over the edge from bad reversing.
Oh, and the obvious, learn the rules of bouys and marine traffic, some harbours require you to have a registration number and if you do something wrong they send you a ticket much like the parking man.
Had this problem with some jet skis once, didn't know the harbour master had an office on top of the hill with a big telescope.