slow water pick up

installed a water pump on my 2012 250 pro xs .It is slow to pick up water but seam to have decent pressure at wot 20psi.any idea why it would be slow to pick up water. I did not have the piece of plastic that sets the depth of the rubber bushing on the shaft.i don't want to hurt my motor so any help would be appricated

Could have trash in thermostats

Could be the thermostats or the poppet needs service. http://www.boats.net/parts/search/Merc2/Mercury/250%20PRO%20XS%20%283.0L%20DFI%29/1B426561%20THRU%201B752546/Exhaust%20Plate/parts.html

Thermostats catch all kinds of junk and will corrode up as will any metal (dissimilar metal corrosion) Pull it/them out and clean them and check for proper operation. The poppet valve works off pressure as well, so do not rule out. A couple years ago I was dealing with bouncing water pressure that turned out to be a bad gasket between the mount halves.