Depends on the boat...
Some, just the little kit that comes when you buy a new outboard is plenty.
Typically: Plug wrench, pliers, screwdriver and a couple of open end box wrenches (appropriate for the engine's normal maintenance)
Add a 8 inch crescent wrench and that should be all a small aluminum boat with maybe up to 30 hp needs.
The fire extinguisher is a REQUIREMENT for any boat with an engine in the US (and most other places) A second extinguisher is not a bad idea.
As the boat gets bigger and more complicated... you need more.
Its a good idea to look around your boat and find what sizes/types screwdrivers, wrenches and other tools you may need for minor repairs. A spare of each size fuse and light bulb would also be a good idea.
There is no single list that will apply to everyone.