Is it recommended to get a life raft for short overwater operations?
I used to be a bush pilot flying Cessna 180s on floats. If you ditch in a 172 on wheels with a life raft in the back seat, it's going to flip inverted when it contacts the water and start to sink immediately, and getting your own carcass and your passenger out of the thing without drowning will be enough of a challenge.  Forget about trying to pull a life raft out of the back seat of a sinking inverted 172 after you just escaped.  It's gone.

If you are that wary of overwater flight, the normal practice, for things like trans Atlantic ferries, is to wear an immersion suit. With the water at 64F, you could get by on a warm weather immersion suit or a sailing dry suit plus life jacket, but even without, it takes quite a few hours for 64F water to kill you, and the Coast Guard is going to be there within 30 min.

In any case, a 172 has an L/D of about 9, which means from 10000 ft you can glide somewhere around 17 miles.  At mid point between between Long Beach and the island you are less than 15 miles from shore, so there's really no need to worry about ditching in the open ocean, unless you want to do the flight at a couple thousand feet.  

Even if you only climb to 5000, you could glide to a few miles of either shore.  And the actual "risk exposure" time of the engine spontaneously blowing up in your face without warning, while out of gliding distance of land, is only a few minutes. The probability of this happening, that is the engine doing something like throwing a rod suddenly, in that little time window, is a fraction of the probability of driving your car off a bridge into the water on your way to the airport.