can anyone tell me how long a water pump impeller last. my mechanic told me it is a ware part. and how hot does the temp guage get when you are idling mine is about 150 to 165 degrees, is that to hot.

mercury 150 EFI.

never had a temp gauge on my OB so i dont know.  as long as it is peeing a good stream of water  at an idle your ok. dont run your engine with out some frog legs and only have the water hose on low and increase the flow if you can not touch it . As far as what you mech. said its true. You should check it for wear and replace as needed. Some say do it every year but it depends on use. I replaced mine about every 3 years because i only got about 10 hrs on it every summer.

Its not hard to check or replace but read up before you do it. Oh you will need a gasket every time you open it up.

I run the hell out of mine (about 100 hours a year) and replace mine and grease the drive shaft spline every two - three years but change the gear oil every June and December.  Since I fish more than several times a month year round, I don't winterize so I go through my standard maintenance every six months.

Ooops!!, You said a Mercury, I'm not sure black motors will run 100 hours without breaking so you can have it done when it's in the shop each year.

The water temp is going to be based the thermostats and I'm not sure what stats mercs run.  I think 165 is getting right on the max of what you would want to see.  Mine (JohnnyRudes) generally run about 130 - 135 at idle under any condititons  and 150 - 155 at WOT on hot summer days.  You could also have a sending unit or guage reading a little high.