Fault lies in several areas. First, the driver is expected NOT to back his vehicle without clear vision or a spotter if he does not have clear vision. However, there are several other areas of "fault" while living in the great People's Republic of California. Here is a short list of the folks who might be included in the lawsuit:
1) The truck manufacturer - for failing to include specific trailer backing instructions in the owner's manual.
2) Any prior owner - in the event that the vehicle owner's manual DID discuss such safety procedures and a prior owner failed to include the manual with the vehicle, a "good" attorney might name the prior owner in the lawsuit.
3) The boat and trailer manufacturer - they undoubtedly carry liability because the trailer failed to provide any signalling device to warn idiots who might stand in the way,
4) The local Parks Department - of course, the ramp lanes were not properly marked or old markings are barely readable,
5) Sherman Williams Paint company - for failing to make a sufficiently durable paint to withstand the elements.
5) Any nearby adult - if you witnessed the incident but failed to act to prevent the collision.
6) The US Post Office - for their failure to develop a "water and trailering safety stamp".
7) The CA Department of Motor Vehicles for failing to license Morons who stand the the path of motor vehciles.

The "victim's" parents - for contributing idiot DNA to the gene pool.
9) All California's Jet Ski owners....just on basic principle!
The lawsuit should be worth millions!
.....NaCl