Wifey B: Williams Jet Tenders now has a MiniJet. It's 9'2", has a 50 hp BRP Rotax Jet, Seats 3 and top speed of 38 mph and it only weighs 465 lbs.
Haven't seen one but their previous smallest was 9'6" and weighed 638 lbs with a Textron 80 hp weighed 695 lbs.
Oh, smallest diesel I'm aware of is from Ribjet and weighs 710 lbs. so beyond the weight levels being discussed.