All I can think of is the obvious stuff...TEST DRIVE,check the oil,tranny fluid ,drive without the stereo on,pay attn to how the boat steers,which way it turns in reverse,all the lights,gauges.If it has ballast,make sure it works.Drive with the ballast full & empty.Make sure there is nothing broken on interior(hinges on storage doors or cupholders,also windshield support arms).Mine were busted.Take all of the seat cushions up,look 'em over, & under them as well.Also check the trailer lights,tires brakes,fluid.
Anyone else?Hope that helps.
Todd