Yeah, Cody owned the Riptide, and the Ebb Tide, Nick owned the Screaming Mimi and a classic Corvette. Given how, in the show, they were always short on cash and in danger of getting behind on their bills, there's no way they should have been able to afford those things. If Cody had inherited the yacht and speed boat that might explain it, otherwise there's no way he should have been able to afford a 1939 Elco 53' cabin cruiser, plus a speed boat. Nick shouldn't have been able to afford the chopper either. Even if he could have bought it cheap because it had been sold as surplus by the military, a helicopter is expensive to maintain, and they have to be inspected annually by the FAA and certified as airworthy -- and there's no way a chopper that sputters and smokes, frequently doesn't start, and occasionally has bits spontaneously fall off the way they did in a couple of episodes would pass inspection. This was a bit of license the show's writers took with realism, rather like the guys having the means to own these toys.
But this is very frequently done in TV and movies. Characters quite often have much nicer cars, houses, apartments, or other toys than people in their jobs should be able to afford. It's a way of making them cooler.
Apparently not. He made money, but evidently it wasn't all that much. Halfway through the first season (which I recently rewatched on DVD) at the beginning of one episode they are playing scrabble, and mention that they haven't had a case or a helicopter tour in a month, and only Murray's royalty money was able to stave off the bill collectors. So whatever he was making was apparently barely enough to cover their living expenses when all other sources of income dried up.
And remember, Cody already had the boats, and Nick his chopper and classic Corvette well before Murray joined the team, and even before they became P.I.s in the pilot episode.