You shouldn't have any problems buying used if you're smart about it. Get it thoroughly check out by a marine mechanic, etc etc. People buy used cars all the time, some have good luck, some have bad luck, some say screw the luck and do the legwork to make sure they don't buy a lemon.
Down here insurance is not required I don't believe. You'd be smart to have insurance just in case. And if it's not required, it d**n well should be. Hell, I'd love to see the state implement required marine drivers licenses.