Sounds like you got an aftermarket manual. In general, those pubs are ok for general information (only), but are otherwise basically doorstops.
You can view/download a factory parts catalog at
http://internetoutboards.com/Parts ...arts Catalog NS5B (2003) - (002N21005-6).pdf
You can also get the service manual from any Tohatsu/Nissan dealer.
Yes, you need to remove the powerhead. 6 bolts, with 10mm heads. The LU can stay on. You will also want 2 new o-rings for the shift lever, possibly the shift lever bushing, and a base gasket for the powerhead. Be careful that you don't lose the spring and ball for the shift detent.
HTH