Change your tstat and impeller. If you just bought the boat you have no idea how old they may be. In cold water and running the tstat does not need to be open much to maintain temp. Then when you idle down with all the extra cold water flowing thru the tstat housing the tstat may remain barely open while the block can act as a heat sink sending the block and temp sensor upward. With low idle impeller flow it can take 10 or more secs depending on impeller flow to get enough water in to cool the block. If this is the case a new impeller and or tstat may be the first things to freshen up. If you find it creeps up after bringing the boat to idle after med to fast run, rev the engine in neutral and the temp should come back down in a couple of seconds.