I bought my first bass boat in August 2019 and I try to go out 2-3 times a month. It's an older 88 Ranger 374v with a 98 Evinrude Intruder. When I bought the boat the lower unit was brand new and the upper unit was rebuilt 6 months prior. How often should I have my lower/upper unit oil changed? Impellers? Spark plugs or any other maintenance you guys would suggest? Thanks

once a year and no more than 2yrs

LU Oil is so cheap I really recommend my customers do it yearly regardless of hours. Impeller yearly unless not boat is not used often, can go 100 ish hours, can stretch 2-3 years (yes, I am aware of the my brother's uncle's step sister has NEVER changed their impeller stories... doesn't mean you want to be that story). Mercury however is advising 300 hours between impeller changes these days.

As for your particular motor, it has a fairly robust impeller, with thick blades. You could get away with a few years if you don't use it but every few weekends.

I have a 96 Johnson, I change my lower unit oil every spring, as said above.. oil is cheap. So why not. I change the water pump every 2 years. I fish a lot. When I have the water pump replaced my guy goes through all the basics. I have ran the same plugs for 4 years before. I run nothing but non ethanol fuel and XD 50 synthetic blend oil. Plugs always look good. I have a compression check done on those 2 year checks also. 25 year old motor. Still good compression on all cylinders.

Thanks for the info guys.