sfm, welcome aboard.
Maxum's owners manuals didn't provide much regarding service. A 1900SR is mainly the engine and outdrive, therefore a aftermarket mercruiser manual is all you need. You can find them online or at any boat store near you.
Do you know when the impeller (water pump) was last replaced? Should be ~ 2 years. Lack of maintance here is what kills most engines. The next thing to inspect is the exhaust as it is water cooled which over time can become clogged or rust through also killing the engine. Other than that its change engine oil and gear lube once a year plus proper winterization.
Good luck and enjoy
1997 2400 SCR
5.7 Vortec / Bravo 2