Re: water pump on 1988 40 HP Nissan
Pretty straightforward job; Disengage shifter, drop LU, unbolt pump (don't lose drive key for impeller), replace parts as needed.
I would make 3 recommendations:
1 - Get a service manual from any dealer (labor cost saved on the w/p job alone will pay for it, and the job will go faster/easier.
2 - Install a water pump kit instead of just the impeller. Same labor, and replaces worn plate and cup along with the impeller.
3 - At this motor age (20+ years), verify and/or replace LU seals while the LU is down. If replacing seals, don't ignore the shifter o-rings.