Granted probably not the case in this instance, but for me it usually starts with forgetting the drain plug.

Hey, I won lots of money as a young marina-monkey in high-school off those drain-plug deficient owners. Faced with a $250 yank-out and dry fee, I'd offer to empty their boat for forty bucks as long as the spark plugs weren't underwater.

I'd push it out of the slip, jump-start it and then run it balls-out across the lake. Physics would drain the hull as it planed out above the water-line and then I'd stick in the drain plug, return it to the slip and collect my cash.

Yeah, I have drained one that way myself. I finally started hooking the plug to the key chain. No plug, no start. I always make sure it will at least crank BEFORE I get to the ramp.

Watched a 40 footer go down leaving the slip into the channel because the bozo forgot to put the drain plugs back in. Nothing but mast sticking up. Bad news is he made getting out of the marina a lot tougher. I mean, it's hard to steer the boat when your on your back pointing and laughing.

other common failures are the water hoses to the engines for cooling, especially if it has water cooled exhaust.
It might have been a failed bilge pump too.
A lot of these boats leak constantly around things like the prop shaft and it is the bilge pumps that keep them on the surface.

I think it is a good bet that if the life raft was never inspected there would be other items that missed regular inspections too.
Rubber hoses rot.

A lot of these small sail cruisers have through hull fittings installed that are incompatable galvanically. As soon as they are in the salt water the different metals start to corrode each other. This could be as simple as somebody cheaping out and buying a plumbing fitting in a hardware store that is copper or even steel instead of bronze.

Quoting from Wikipedia;
" Galvanic series tables for seawater are commonplace due to the extensive use of metal in shipbuilding. It is possible that corrosion of silver brazing in a salt water pipe might have caused a failure that lead to the USS Thresher sinking with all men lost."
http://en.wikipedia.org/wiki/Galvanic_corrosion

Check you water heater fittings. That is where the cheaping out usually occurs. Won't generally cost you your life, but one heck of a lot of money. This is where I learned about dissimilar metals and galvanic reactions.