Sounds like maybe there's something either in the pipe from the pump, or maybe the valve isn't open all the way? When you re-did all the hoses did you make sure you hooked them up right? What other changes did you make?

Sounds like you have all the bases covered, have you made sure that there's no blockage at the pump?

 No thermostat in the engine and all water goes out the exhaust manifolds.

I have a buddy that has a 1999 Ultra with a Marine power 454. All stock except a MSD box on the stock dizzy. When he first bought the boat last year the temp. gauge ran at 140 all day, now it is showing 180 and up. We cleaned out alot of sand from the water jackets and replaced all the water hoses on the engine but still ran hot, if he stays under 2,800 to 3,000 RPM all is OK but if you go over that it start to heat up! What else should we look at? 

Assuming no guage problems (false readings etc) Quite honestly considering all you have indicated I'd start thinking towards potential internal jet drive "issues" if the pump feeds the motor with no interfering valving and the exaust system gets the water last the possibility exists wear in the jet is responsible Any changes in performance (rpm's vs speed) Tom

There was sand in the water jackets in the block, back flushed thru the drain holes on lower side of engine block, made sure all sand was out. Tom , how would I check out the pump issue you talked about the friend dose not remember any RPM droops or speed but he can't be sure.

Fact is Jet boats just don't run hot! Very unusual in fact the biggest complaint is usually that they wont get off the cold peg. #1 cause of jet boat overheating in our world is worn Jet Drive internals , second is incorrect plumbing (I'm guessin this has center rize ex mans ?) Truth be known if the "tune ups right" (no lean out or timing isues) and the water coming off the jet is unretricted this boat should run 100 to 130 all day long (ambient and water temps 60 to 80's) The Jet is the water pump theres not much else , Water usually blasts thru these motors like crap thru a goose . The best test for the Jet drive is RPMs vs speed soooo if all thats good theres got to be an obstruction BUT not likley . Maybe give some thought to "back flushing" (air or water) all the water lines Call me if you need more help Happy to be of assistance Tom

36 degrees total!