Re: 90A Nissan Shift lever release
First, try to get a Factory service manual from a Nissan/Tohatsu dealer. That will show you a lot. Failing that, you can get a parts catalog online at Elvin's site,
http://www.internetoutboards.com/PartsCatalogMenu.htm Yes, after you disengage the shifter and unbolt the LU, it can be taken down, exposing the water pump. I would recommend getting a WP kit, Part # 3B7873221M List Price (MSRP): $86.67, as this will include not just the impeller, but also the cup it rides in, the wear plate, drive key, and associated gaskets, etc.
Carb work is both a science and an art. If you are not at all familiar with the technology, better to find someone who is... perhaps a dealer. A multiple-carb motor will require synchronization of the carbs after carb work... again, best done by someone familiar with how it goes.